docs: add AGENTS.md for Cursor Cloud development setup

[Nu424]

Adds AGENTS.md with Cursor Cloud-specific instructions for developing だしゃろん (dasharon).

Summary

• Documents the single required local service (Vite dev server on port 5173)
• References standard commands from README.md / package.json
• Notes lint pre-existing issues and E2E testing requirements (OpenRouter API key, mic)
• Includes tmux dev-server startup snippet for cloud VMs

Environment verification

• npm install — OK
• npm run build — OK
• npm run lint — 6 pre-existing ESLint errors (unchanged)
• npm run dev — serving at http://localhost:5173
• Browser demo: app loads, Settings modal works, Edit mode toggles

dasharon-dev-environment-demo.mp4

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