docs: add Cursor Cloud dev environment setup notes

[ephraimduncan]

Summary

Sets up and documents the development environment for the Weekday Calendar monorepo (Bun + Turborepo + Next.js 16 + PostgreSQL + Drizzle + Better Auth).

Adds an AGENTS.md with a ## Cursor Cloud specific instructions section capturing the non-obvious caveats for running this project:

• PostgreSQL must run on port 5433 (Docker unavailable here; Postgres installed as a system cluster).
• .env is required and gitignored; which vars are needed and which can be placeholders.
• Login is Google-only — real login needs Google OAuth credentials; app otherwise boots fully.
• Root turbo lint has no lint task; ESLint on this commit throws a circular-JSON error (eslint-config-next@16 + FlatCompat incompatibility, pre-existing).
• No automated test scripts exist.

The suggested startup update script is minimal: bun install.

Verification

Environment was set up and the app run end-to-end in development mode.

weekday_app_walkthrough.mp4

Landing page, login page, and protected-route redirect all working:

Landing page
Login page
/calendar redirects to /login

The Google OAuth sign-in action was exercised via the auth API, generating a real Google authorization URL and persisting the corresponding state to Postgres — proving the app → auth → DB pipeline works.

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[ephraimduncan]

cursor specific?