refactor: move TodoMVC example into example/ directory

- example/todomvc/pocketbook/todomvc.cljs — app source
- example/todomvc/todomvc.html — page
- example/todomvc/js/ — compiled output (gitignored)

Server static file serving is now filesystem-based via --static-dir
flag instead of classpath resources. No static dir by default (API
only); bb server passes --static-dir example/todomvc.

Removed resources/public/ — library has no bundled static assets.

bb.edn

@@ -1,6 +1,6 @@
 {:tasks
- {server {:doc "Start the Pocketbook sync server"
-          :task (shell "clj" "-M:server")}
+ {server {:doc "Start the Pocketbook sync server (serves example app)"
+          :task (shell "clj" "-M:server" "--static-dir" "example/todomvc")}
 
   cljs {:doc "Compile ClojureScript (one-shot)"
         :task (shell "clj" "-M:cljs")}