Clean up and consolidate stale documentation

Use context architecture skill to scrub all old documentation, including instructions, specs, trackers, ADRs - delete everything superseeded and stale, remove references to historical decisions, delete duplicates, single home everything and make sure all documentation is accessible from CLAUDE.md router in a few steps

Audited docs, ADR register, specs, lessons, research and router: already clean; check_context passes, remaining --audit advisories are false positives. No change, no PR. Memory: none applied.