Add Cursor Cloud development environment documentation

[whetherwhichwood]

Summary

Adds root AGENTS.md with Cursor Cloud-specific instructions for developing and testing all three PM-tooling snapshots in this monorepo.

What's documented

• Dependency install and VM update script expectations
• Validation commands for all three projects (bash/Python)
• AI PM dashboard dev server, tests, and build
• Streamlit playground startup and API key requirements
• tmux patterns for long-running Vite/Streamlit sessions
• Common gotchas (catalog regeneration, tsc build artifacts)

Verification performed

• bash tests/check-skills.sh — 120 checks passed
• python3 validate_plugins.py — 9 plugins, 110 components passed
• ./scripts/test-library.sh — skills/commands validated
• npm test -- --run — 65 Vitest tests passed
• npm run build — production build succeeded
• Vite (:5173) and Streamlit (:8501) both serving HTTP 200
• Manual UI demo: orchestrator workflow + Skill Finder search