Expand README: ask Cursor to run Python + full install guide

[kazuyamagiwa]

Summary

Expands README.md into a full guide for the nested cursory/ install kit.

Highlights

• Ask Cursor to run Python — document that after install you can just ask Cursor to run scripts, tests, or sync; the agent follows AGENTS.md and uses ./scripts/run-python.sh
• Example prompt → command table
• Why / prerequisites / install steps / installer behavior
• Templates map, rules, day-to-day commands, deps, secrets
• Cleanup rules, post-install layout, troubleshooting
• Keeps topic badges and Contents anchors