Add AGENTS.md with Cursor Cloud development environment instructions

[tetsup]

Summary

Adds AGENTS.md with Cursor Cloud specific instructions for setting up and developing in this codebase.

What's included

• Prerequisites documentation (Node.js >= 24, pnpm 10.33.0, bun for tsdx lint)
• Key commands reference table (lint, typecheck, test, build, dev server, E2E)
• Known gotchas discovered during setup:
  • tsdx lint requires bun on PATH (not obvious since it's a pnpm project)
  • Latest oxlint (v1.x) fails parsing vite.config.ts; pinned workaround documented
  • SharedArrayBuffer COOP/COEP headers requirement
  • E2E test prerequisites (build:test must run first)
  • Test game requires clicking "Start" button to begin rendering

Environment verification

All checks passed during setup:

• ✅ pnpm test — 6/6 unit tests pass
• ✅ pnpm typecheck — clean
• ✅ pnpm build — library builds successfully
• ✅ pnpm build:test — test game builds
• ✅ pnpm test:e2e — 1/1 E2E test passes
• ✅ pnpm dev — Vite dev server runs at localhost:5173
• ⚠️ pnpm lint — tsdx lint uses bunx oxlint@latest which fails on vite.config.ts (pre-existing upstream issue); bunx oxlint@0.16 src works cleanly

Demo

web2d_game_demo.mp4

Game running with sprite rendered on canvas after clicking Start:

Game running with sprite visible

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