Set up Cursor Cloud dev environment (AGENTS.md + update script)

[dddiggory]

Summary

Sets up the development environment for the Cursor Cookbook so cloud agents can install, lint, build, and run the SDK examples.

• Adds AGENTS.md with durable ## Cursor Cloud specific instructions (layout, per-example commands, Bun requirement, and the CURSOR_API_KEY gotcha).
• Configures the startup update script to install Bun (guarded) and run pnpm install for each sdk/* example.

No application code changed.

Environment

• Node 22 + pnpm preinstalled; Bun installed for sdk/coding-agent-cli.
• Each sdk/* example is its own pnpm workspace; installed per-directory.

Verification

Example	Checks run
sdk/app-builder	pnpm lint, pnpm build, pnpm dev (booted, onboarding UI)
sdk/agent-kanban	pnpm lint, pnpm build
sdk/quickstart	pnpm typecheck, pnpm build
sdk/dag-task-runner	pnpm typecheck, pnpm build, pnpm init-canvas
sdk/coding-agent-cli	pnpm typecheck

Hello-world demos:

• sdk/dag-task-runner pnpm init-canvas renders the DAG canvas file (no key needed).
• sdk/app-builder dev server serves the onboarding flow; entering a key triggers real backend validation against the Cursor API.

Note: full agent runs require a valid CURSOR_API_KEY secret; the environment's FE_ADM_API_KEY is not a valid SDK key.