docs: add AGENTS.md with Cursor Cloud dev setup notes

[darknessomi]

Summary

Sets up the development environment for this YApi source deployment and documents the non-obvious cloud caveats in a new AGENTS.md.

Verified end-to-end in the cloud VM:

• Installed dependencies (npm install, Node v22 which satisfies engines: node >= 20).
• Installed & ran MongoDB 6 locally, created the config.json (gitignored) pointing at 127.0.0.1, and initialized the DB / admin via server/install.js.
• Ran the two dev processes (npm run dev-server on :3000, npm run dev-client on :4000).
• Hello-world: logged in as admin@admin.com and created a project "Hello World API".

Notable gotcha captured in AGENTS.md: server/yapi.js resolves WEBROOT_RUNTIME to the filesystem root / in a source checkout, so /log and /init.lock live at / and need write access.

Testing

• ✅ npm run lint
• ✅ npm test (60 passed)
• ✅ npm run dev-server + npm run dev-client (app reachable at http://127.0.0.1:3000)
• ✅ Manual: admin login + create project

yapi_login_create_project.mp4

Hello World API project created

The update script is npm install (snapshot-managed environment; MongoDB start and config.json are documented in AGENTS.md rather than the update script).

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