Overhaul Repopo Documentation

[tylerbutler]

Public documentation is significantly lagging behind internal knowledge and implementation.

Action Items:

• Rewrite usage/config.mdx (currently a copy-paste error of CLI usage).
• Sync architectural documentation from packages/repopo/CLAUDE.md to the public README and docs site.
• Add a "Quick Start" guide for new users.
• Update all code examples to use the modern policy() API instead of makePolicy.