docs: add Cursor Cloud dev environment setup notes

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+Weekday is a single-product Bun + Turborepo monorepo: a Next.js 16 web app
+(`apps/web`, the only runnable service, on port 3000) backed by PostgreSQL via
+Drizzle ORM, tRPC, and Better Auth. Packages under `packages/*` (`api`, `auth`,
+`db`, `env`, `google-calendar`, `lib`) are libraries consumed by the web app.
+
+Standard commands live in the root `package.json` and `readme.md`; prefer those.
+The notes below are only the non-obvious caveats for this environment.
+
+### Services
+
+- PostgreSQL must be running on port **5433** (not the default 5432) to match
+  `DATABASE_URL` in `.env` and `docker-compose.yml`. Docker is not available
+  here; Postgres 16 is installed as a system cluster instead. Start it with
+  `sudo pg_ctlcluster 16 main start` (the cluster is preconfigured for port 5433
+  and the `postgres` user password is `password`). It does not auto-start on
+  boot.
+- Run only the web app with `bun run dev:next` (filters to `@weekday/web`).
+  Plain `bun run dev` also works but only `@weekday/web` has a `dev` task.
+
+### Environment file
+
+- `.env` is gitignored and required (env schema in `packages/env/index.js`
+  validates it at startup). If it is missing, copy `.env.example` to `.env` and
+  set: `DATABASE_URL=postgresql://postgres:password@localhost:5433/postgres`,
+  a generated `BETTER_AUTH_SECRET` (`openssl rand -base64 32`), and
+  `BETTER_AUTH_URL=http://localhost:3000`. `BETTER_AUTH_GOOGLE_ID`,
+  `BETTER_AUTH_GOOGLE_SECRET`, and `AI_GATEWAY_API_KEY` are required by the
+  schema (non-empty), so use placeholders to boot; real values are only needed
+  for actual Google login / AI features.
+- After `.env` exists and Postgres is up, run `bun run db:push` to sync the
+  Drizzle schema (creates the `weekday_*` tables). This is not part of the
+  startup update script.
+
+### Auth caveat (login is Google-only)
+
+- The only sign-in method is Google OAuth (`/login` -> "Login with Google").
+  Completing a real login requires valid `BETTER_AUTH_GOOGLE_ID` /
+  `BETTER_AUTH_GOOGLE_SECRET` from a Google Cloud OAuth client whose redirect
+  URI is `http://localhost:3000/api/auth/callback/google`, plus a Google
+  account. With placeholder credentials the app boots and all public routes and
+  the auth API work (the sign-in endpoint generates a real Google OAuth URL and
+  writes state to the DB), but the OAuth round-trip cannot complete.
+
+### Lint
+
+- `turbo lint` / root `bun run lint` fails: `turbo.json` defines no `lint` task.
+  Run lint per package instead (e.g. `cd apps/web && bun run lint`).
+- On this commit `apps/web` ESLint itself throws
+  `Converting circular structure to JSON` — a known incompatibility between
+  `eslint-config-next@16` and the `FlatCompat` layer, unrelated to environment
+  setup.
+
+### Tests
+
+- There are no automated test scripts in this repo.