Add AGENTS.md

AGENTS.md

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+# OpenPencil
+
+Vue 3 + CanvasKit (Skia WASM) + Yoga WASM design editor. Tauri v2 desktop app, also runs in browser.
+
+## Commands
+
+- `bun run check` — lint + typecheck (run before committing)
+- `bun run format` — oxfmt with import sorting
+- `bun test ./tests/engine` — unit tests
+- `bun run test` — Playwright visual regression
+
+## Rules
+
+- Use `@/` import alias for cross-directory imports, `./` for same directory
+- No `any` — use proper types, generics, declaration merging
+- No `!` non-null assertions — use guards, `?.`, `??`
+- Shared types live in `src/types.ts` (GUID, Color, Vector, Matrix, Rect)
+- Mac shortcuts: use `e.code` not `e.key` (Option transforms characters)
+- UI components: use reka-ui
+- `src/kiwi/kiwi-schema/` is vendored — don't modify