docs: add AGENTS.md with Cursor Cloud development instructions

[tonyketcham]

Summary of changes

Adds AGENTS.md with Cursor Cloud specific development instructions, documenting key setup gotchas for autonomous cloud agents working on this codebase:

• Native build scripts blocked by pnpm v10 (esbuild, sharp, SWC, Tailwind oxide)
• Vitest watch mode pitfall (use vitest run)
• flatbread CLI not on PATH (use npx flatbread)
• Build-before-test requirement
• HTTPS flag considerations for headless environments

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• I added doc comments to any new public exports, and inline comments to any hard-to-understand areas
• My changes generate no new console errors locally
• If applicable, try to include a test that fails without this PR but passes with it

Does this introduce any non-backwards compatible changes?

• Yes
• No

Does this include any user config changes?

• Yes
• No