/sync-docs: find drift

1. Compare:
   • requirements ↔ feature specs: every F-xx has a spec or a "not written" entry
   • feature specs ↔ tasks ↔ roadmap: milestones and statuses agree
   • ADRs ↔ docs: accepted decisions are reflected; nothing contradicts them
   • docs ↔ code (from M0): structure, commands, API, data model
   • links: every relative link resolves
   • public voice: no personal or raw notes in public docs
2. Report a short list: file: problem → suggested fix. Most severe first.
3. Fix only after the owner agrees, and only what they approve. Don't commit.