Setup AGENTS.md and CLAUDE.md (#4147)

AGENTS.md

@@ -1,3 +1,77 @@
 # AGENTS Instructions
 
-- After making code changes, run `bun format`.
+## Commands
+
+```bash
+bun install          # Install dependencies
+bun dev              # Start dev server (all packages)
+bun run build        # Build all packages
+bun run format       # Format with Biome (run after every change)
+bun run typecheck    # Type-check all packages
+bun run unit         # Run unit tests
+```
+
+## Development
+
+The dev server proxies published GitBook sites locally. After `bun dev`, access any published site at:
+
+```
+http://localhost:3000/url/<published-gitbook-url>
+```
+
+Examples:
+- `http://localhost:3000/url/gitbook.com/docs`
+- `http://localhost:3000/url/open-source.gitbook.io/midjourney`
+
+## Architecture
+
+```
+packages/
+  gitbook/          # Main Next.js app
+    src/
+      app/          # Next.js App Router (sites/)
+      components/   # React components
+      lib/          # Server utilities, data fetching
+      intl/         # Internationalization (translations/)
+  openapi-parser/   # OpenAPI 3.0/3.1/Swagger parser
+  react-openapi/    # OpenAPI rendering components
+  react-contentkit/ # ContentKit component rendering
+  embed/            # Embeddable GitBook components
+  shared/           # Shared utilities
+  icons/            # Icon assets
+  fonts/            # Font assets
+  colors/           # Color tokens
+  expr/             # GitBook expression evaluator
+  cache-do/         # Cloudflare DO cache
+  cache-tags/       # Cache tag utilities
+```
+
+## Testing
+
+```bash
+bun run unit         # Unit tests via bun test (not vitest)
+bun run e2e          # Playwright e2e tests (requires built app)
+```
+
+Run a specific test file:
+```bash
+cd packages/gitbook && bun test src/lib/cache.test.ts
+```
+
+## Changesets
+
+After committing code changes, create a changeset for the affected package:
+
+```md
+---
+"gitbook": patch
+---
+
+Provide a short description of the change.
+```
+
+Save as `.changeset/<name>.md`, then commit it separately with message: `changeset`
+
+## Formatting
+
+Uses Biome (not ESLint/Prettier). Always run `bun run format` before committing.

CLAUDE.md

@@ -0,0 +1 @@
+@AGENTS.md