docs: add AGENTS.md with async conventions and project structure

AGENTS.md

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+# Pocketbook
+
+Clojure/ClojureScript offline-first synced atom library backed by IndexedDB (CLJS) or SQLite (CLJ).
+
+## Async: use promesa, not core.async
+
+This project uses **promesa** (`funcool/promesa`) for all async operations. Do not introduce `core.async`.
+
+- **Store protocol** (`store.cljc`): all methods return promesa promises
+  - Sync operations (e.g. MemoryStore): `(p/resolved val)`
+  - Async operations (e.g. IDBStore): `(p/create (fn [resolve reject] ...))`
+- **HTTP / sync** (`sync.cljc`): CLJ uses `p/vthread`, CLJS returns native JS Promise chains
+- **Core** (`core.cljc`): `p/let` for sequential async, `p/all` for parallel
+- **Sync loop**: platform timers (`setInterval` / `ScheduledExecutorService`), not go-loop
+- **Push coalescing**: debounced with 50ms timer + `pushing?` atom guard, not channels
+
+### Why not core.async
+
+- Channels-as-one-shot-promises add ceremony (`chan` + `put!` + `close!`) for what is conceptually a return value
+- promesa maps to native primitives: JS Promises on CLJS, CompletableFuture on CLJ
+- `p/let` reads cleaner than `go` + `<!` for sequential async
+- Timer-based sync loop is simpler than `go-loop` + `alts!` for this use case
+
+## Project structure
+
+```
+src/pocketbook/
+  store.cljc        — PStore protocol (promises)
+  store/memory.cljc — in-memory store (testing/JVM)
+  store/idb.cljs    — IndexedDB store (browser)
+  sync.cljc         — HTTP pull/push, SSE, connectivity
+  core.cljc         — SyncedAtom, sync loop, public API
+  transit.cljc      — Transit encoding/decoding
+  db.clj            — SQLite server storage (nippy)
+  server.clj        — HTTP sync server (http-kit)
+```
+
+## Running tests
+
+```
+clj -M:dev:test
+```