Clean up and organize documentation with router

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 with the context-architecture checker and manual sweeps; the tree is already clean, so no change and no PR. Memory: none applied.