chore: adopt compound-engineering cursor conventions

[daopunk]

Summary

• Adopt Compound Engineering repo conventions for Cursor: artifact folders (docs/plans, docs/brainstorms, docs/solutions, todos), CE config example, and AGENTS.md loop docs
• Move agent commit rules into .cursor/rules/*.mdc and add an always-on compound-engineering rule
• Keep domain skills under .agents/skills/; document marketplace install (/add-plugin compound-engineering) + /ce-setup

Test plan

• Install plugin in Cursor: /add-plugin compound-engineering
• Run /ce-setup and confirm project config is healthy
• Confirm .cursor/rules/*.mdc appear under Cursor Rules
• Confirm domain skills still listed under Customize → Skills
• Smoke: /ce-brainstorm or /ce-plan writes under the new docs/ artifact folders without colliding with product docs