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 all docs (ADR register, specs, trackers, lessons, research, duplicates, reachability); the tree is already clean and check_context passes. Nothing stale to delete, so no PR.