refactor: replace BEM with utility classes, move component CSS to files

- Rename CSS classes from BEM double-dash (btn--primary) to flat
  utility-style single-dash (btn-primary)
- Move button CSS from inline Clojure string in gen.clj to
  src/ui/button.css next to button.cljc
- gen.clj now auto-collects all src/ui/*.css files via babashka.fs glob
- Replace clojure.java.io with babashka.fs throughout gen.clj
- Update AGENTS.md to reflect new conventions

AGENTS.md

@@ -10,6 +10,7 @@ src/
   ui/
     theme.cljc               # Token helpers (css-var)
     button.cljc              # Button component (reference implementation)
+    button.css               # Button component styles (read by gen.clj)
     css/gen.clj              # EDN → CSS generator (babashka)
 dist/
   theme.css                  # Generated CSS (tokens + component styles)
@@ -126,33 +127,27 @@ Key rules:
 - **`:squint` branch**: `:class` is a string, `:style` is a string-keyed map, events are flat (`:on-click`)
 - **`:clj` branch**: `:class` and `:style` are both strings, no event handlers
 
-### 2. Add CSS to `src/ui/css/gen.clj`
+### 2. Add CSS file `src/ui/COMPONENT.css`
 
-Add a `component-css-card` function and include it in `generate-css`:
+Create a plain CSS file next to the `.cljc` file. The generator automatically reads all `.css` files from `src/ui/`:
 
-```clojure
-(defn component-css-card []
-  (str/join "\n\n"
-    [".card {
+```css
+/* src/ui/card.css */
+.card {
   padding: 1rem;
   border-radius: var(--radius-md);
   background: var(--bg-1);
-}"
-     ".card--elevated {
-  box-shadow: var(--shadow-1);
-}"]))
+}
 
-;; In generate-css, add to the components list:
-(defn generate-css [{:keys [tokens themes]}]
-  (let [...]
-    (str/join "\n\n" [root-block dark-attr dark-media
-                      (component-css-button)
-                      (component-css-card)  ;; <-- add here
-                      ""])))
+.card-elevated {
+  box-shadow: var(--shadow-1);
+}
 ```
 
+No changes needed in `gen.clj` — it collects all `src/ui/*.css` files automatically.
+
 CSS conventions:
-- BEM-lite: `.component`, `.component--variant`, `.component--size`
+- Utility-style flat classes: `.component`, `.component-variant`, `.component-size`
 - Use `var(--token-name)` for all colors, borders, shadows, radii
 - Include hover/focus/disabled states
 - Keep specificity flat — no nesting beyond `:hover:not(:disabled)`
@@ -168,9 +163,9 @@ Test the pure class-generation functions (they run in `:clj` via Babashka):
 
 (deftest card-class-list-test
   (testing "default variant"
-    (is (= ["card" "card--default"] (card/card-class-list {}))))
+    (is (= ["card" "card-default"] (card/card-class-list {}))))
   (testing "explicit variant"
-    (is (= ["card" "card--elevated"] (card/card-class-list {:variant :elevated})))))
+    (is (= ["card" "card-elevated"] (card/card-class-list {:variant :elevated})))))
 ```
 
 Register new test namespaces in `bb.edn`:

src/ui/button.cljc

@@ -11,12 +11,12 @@
 
 (defn button-class-list
   "Generate a vector of CSS class strings for a button given variant and size.
-   Returns e.g. [\"btn\" \"btn--primary\" \"btn--lg\"]."
+   Returns e.g. [\"btn\" \"btn-primary\" \"btn-lg\"]."
   [{:keys [variant size]}]
   (let [v (or (some-> variant kw-name) default-variant)
         s (or (some-> size kw-name) default-size)]
-    (cond-> ["btn" (str "btn--" v)]
-      (not= s "md") (conj (str "btn--" s)))))
+    (cond-> ["btn" (str "btn-" v)]
+      (not= s "md") (conj (str "btn-" s)))))
 
 (defn button-classes
   "Generate CSS class string for a button. Returns a space-joined string."

src/ui/button.css

@@ -0,0 +1,65 @@
+.btn {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  gap: 0.5em;
+  padding: 0.5rem 1rem;
+  font-size: 0.875rem;
+  font-weight: 500;
+  line-height: 1.25rem;
+  border: none;
+  border-radius: var(--radius-md);
+  cursor: pointer;
+  transition: background-color 0.15s ease, box-shadow 0.15s ease, opacity 0.15s ease;
+  font-family: inherit;
+}
+
+.btn-primary {
+  background: var(--accent);
+  color: var(--fg-on-accent);
+}
+.btn-primary:hover:not(:disabled) {
+  filter: brightness(1.1);
+}
+
+.btn-secondary {
+  background: var(--bg-1);
+  color: var(--fg-0);
+  border: var(--border-0);
+}
+.btn-secondary:hover:not(:disabled) {
+  background: var(--bg-2);
+}
+
+.btn-ghost {
+  background: transparent;
+  color: var(--fg-0);
+}
+.btn-ghost:hover:not(:disabled) {
+  background: var(--bg-1);
+}
+
+.btn-danger {
+  background: var(--danger);
+  color: var(--fg-on-danger);
+}
+.btn-danger:hover:not(:disabled) {
+  filter: brightness(1.1);
+}
+
+.btn-sm {
+  padding: 0.25rem 0.5rem;
+  font-size: 0.75rem;
+  line-height: 1rem;
+}
+
+.btn-lg {
+  padding: 0.75rem 1.5rem;
+  font-size: 1rem;
+  line-height: 1.5rem;
+}
+
+.btn:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}

src/ui/css/gen.clj

@@ -1,6 +1,6 @@
 (ns ui.css.gen
-  (:require [clojure.edn :as edn]
-            [clojure.java.io :as io]
+  (:require [babashka.fs :as fs]
+            [clojure.edn :as edn]
             [clojure.string :as str]))
 
 (defn read-tokens
@@ -32,68 +32,13 @@
   transition: background-color 0.2s, color 0.2s;
 }")
 
-(defn component-css-button
-  "Generate BEM-lite CSS for the button component."
-  []
-  (str/join "\n\n"
-    [".btn {
-  display: inline-flex;
-  align-items: center;
-  justify-content: center;
-  gap: 0.5em;
-  padding: 0.5rem 1rem;
-  font-size: 0.875rem;
-  font-weight: 500;
-  line-height: 1.25rem;
-  border: none;
-  border-radius: var(--radius-md);
-  cursor: pointer;
-  transition: background-color 0.15s ease, box-shadow 0.15s ease, opacity 0.15s ease;
-  font-family: inherit;
-}"
-     ".btn--primary {
-  background: var(--accent);
-  color: var(--fg-on-accent);
-}
-.btn--primary:hover:not(:disabled) {
-  filter: brightness(1.1);
-}"
-     ".btn--secondary {
-  background: var(--bg-1);
-  color: var(--fg-0);
-  border: var(--border-0);
-}
-.btn--secondary:hover:not(:disabled) {
-  background: var(--bg-2);
-}"
-     ".btn--ghost {
-  background: transparent;
-  color: var(--fg-0);
-}
-.btn--ghost:hover:not(:disabled) {
-  background: var(--bg-1);
-}"
-     ".btn--danger {
-  background: var(--danger);
-  color: var(--fg-on-danger);
-}
-.btn--danger:hover:not(:disabled) {
-  filter: brightness(1.1);
-}"
-     ".btn--sm {
-  padding: 0.25rem 0.5rem;
-  font-size: 0.75rem;
-  line-height: 1rem;
-}"
-     ".btn--lg {
-  padding: 0.75rem 1.5rem;
-  font-size: 1rem;
-  line-height: 1.5rem;
-}"
-     ".btn:disabled {
-  opacity: 0.5;
-  cursor: not-allowed;
-}"]))
+(defn collect-component-css
+  "Read all .css files from the component source directory."
+  [dir]
+  (->> (fs/glob dir "*.css")
+       (sort-by fs/file-name)
+       (map #(slurp (str %)))
+       (str/join "\n\n")))
 
 (defn generate-css
   "Generate the full CSS output from parsed token data."
@@ -106,7 +51,7 @@
                          (str/replace (tokens->css-block dark-tokens) #"(?m)^  " "    ")
                          "\n  }\n}")
         base        (base-css)
-        components  (component-css-button)]
+        components  (collect-component-css "src/ui")]
     (str/join "\n\n" [root-block dark-attr dark-media base components ""])))
 
 (defn build-theme!
@@ -114,6 +59,6 @@
   [{:keys [input output]}]
   (let [token-data (read-tokens input)
         css        (generate-css token-data)]
-    (io/make-parents output)
+    (fs/create-dirs (fs/parent output))
     (spit output css)
     (println (str "Generated " output " (" (count (str/split-lines css)) " lines)"))))

test/ui/button_test.clj

@@ -4,54 +4,54 @@
 
 (deftest button-class-list-test
   (testing "default variant and size"
-    (is (= ["btn" "btn--secondary"] (button/button-class-list {}))))
+    (is (= ["btn" "btn-secondary"] (button/button-class-list {}))))
 
   (testing "explicit variant"
-    (is (= ["btn" "btn--primary"] (button/button-class-list {:variant :primary})))
-    (is (= ["btn" "btn--ghost"] (button/button-class-list {:variant :ghost})))
-    (is (= ["btn" "btn--danger"] (button/button-class-list {:variant :danger}))))
+    (is (= ["btn" "btn-primary"] (button/button-class-list {:variant :primary})))
+    (is (= ["btn" "btn-ghost"] (button/button-class-list {:variant :ghost})))
+    (is (= ["btn" "btn-danger"] (button/button-class-list {:variant :danger}))))
 
   (testing "explicit size"
-    (is (= ["btn" "btn--secondary" "btn--sm"] (button/button-class-list {:size :sm})))
-    (is (= ["btn" "btn--secondary"] (button/button-class-list {:size :md})))
-    (is (= ["btn" "btn--secondary" "btn--lg"] (button/button-class-list {:size :lg}))))
+    (is (= ["btn" "btn-secondary" "btn-sm"] (button/button-class-list {:size :sm})))
+    (is (= ["btn" "btn-secondary"] (button/button-class-list {:size :md})))
+    (is (= ["btn" "btn-secondary" "btn-lg"] (button/button-class-list {:size :lg}))))
 
   (testing "variant + size combined"
-    (is (= ["btn" "btn--primary" "btn--lg"] (button/button-class-list {:variant :primary :size :lg})))))
+    (is (= ["btn" "btn-primary" "btn-lg"] (button/button-class-list {:variant :primary :size :lg})))))
 
 (deftest button-classes-test
   (testing "default variant and size"
-    (is (= "btn btn--secondary" (button/button-classes {}))))
+    (is (= "btn btn-secondary" (button/button-classes {}))))
 
   (testing "explicit variant"
-    (is (= "btn btn--primary" (button/button-classes {:variant :primary})))
-    (is (= "btn btn--ghost" (button/button-classes {:variant :ghost})))
-    (is (= "btn btn--danger" (button/button-classes {:variant :danger})))
-    (is (= "btn btn--secondary" (button/button-classes {:variant :secondary}))))
+    (is (= "btn btn-primary" (button/button-classes {:variant :primary})))
+    (is (= "btn btn-ghost" (button/button-classes {:variant :ghost})))
+    (is (= "btn btn-danger" (button/button-classes {:variant :danger})))
+    (is (= "btn btn-secondary" (button/button-classes {:variant :secondary}))))
 
   (testing "explicit size"
-    (is (= "btn btn--secondary btn--sm" (button/button-classes {:size :sm})))
-    (is (= "btn btn--secondary" (button/button-classes {:size :md})))
-    (is (= "btn btn--secondary btn--lg" (button/button-classes {:size :lg}))))
+    (is (= "btn btn-secondary btn-sm" (button/button-classes {:size :sm})))
+    (is (= "btn btn-secondary" (button/button-classes {:size :md})))
+    (is (= "btn btn-secondary btn-lg" (button/button-classes {:size :lg}))))
 
   (testing "variant + size combined"
-    (is (= "btn btn--primary btn--lg" (button/button-classes {:variant :primary :size :lg})))
-    (is (= "btn btn--danger btn--sm" (button/button-classes {:variant :danger :size :sm}))))
+    (is (= "btn btn-primary btn-lg" (button/button-classes {:variant :primary :size :lg})))
+    (is (= "btn btn-danger btn-sm" (button/button-classes {:variant :danger :size :sm}))))
 
   (testing "nil variant falls back to default"
-    (is (= "btn btn--secondary" (button/button-classes {:variant nil}))))
+    (is (= "btn btn-secondary" (button/button-classes {:variant nil}))))
 
   (testing "nil size falls back to default (md, no size class)"
-    (is (= "btn btn--secondary" (button/button-classes {:size nil}))))
+    (is (= "btn btn-secondary" (button/button-classes {:size nil}))))
 
   (testing "string variants work"
-    (is (= "btn btn--primary" (button/button-classes {:variant "primary"})))))
+    (is (= "btn btn-primary" (button/button-classes {:variant "primary"})))))
 
 (deftest button-component-test
   (testing "basic button renders correct hiccup (clj target)"
     (let [result (button/button {:variant :primary} "Click me")]
       (is (= :button (first result)))
-      (is (= "btn btn--primary" (get-in result [1 :class])))
+      (is (= "btn btn-primary" (get-in result [1 :class])))
       (is (= "Click me" (nth result 2)))))
 
   (testing "disabled button has disabled attr"
@@ -64,7 +64,7 @@
 
   (testing "extra class gets appended"
     (let [result (button/button {:variant :primary :class "extra"} "Test")]
-      (is (= "btn btn--primary extra" (get-in result [1 :class])))))
+      (is (= "btn btn-primary extra" (get-in result [1 :class])))))
 
   (testing "extra attrs get merged"
     (let [result (button/button {:variant :primary :attrs {:id "my-btn"}} "Test")]

test/ui/theme_test.clj

@@ -42,12 +42,12 @@
 
     (testing "contains button component CSS"
       (is (str/includes? css ".btn {"))
-      (is (str/includes? css ".btn--primary {"))
-      (is (str/includes? css ".btn--secondary {"))
-      (is (str/includes? css ".btn--ghost {"))
-      (is (str/includes? css ".btn--danger {"))
-      (is (str/includes? css ".btn--sm {"))
-      (is (str/includes? css ".btn--lg {"))
+      (is (str/includes? css ".btn-primary {"))
+      (is (str/includes? css ".btn-secondary {"))
+      (is (str/includes? css ".btn-ghost {"))
+      (is (str/includes? css ".btn-danger {"))
+      (is (str/includes? css ".btn-sm {"))
+      (is (str/includes? css ".btn-lg {"))
       (is (str/includes? css ".btn:disabled {")))))
 
 (deftest tokens-roundtrip-test