Clean 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

Docs were already near-clean: all 142 ADRs current, no stale specs or trackers, no duplicates, everything routed. Removed task-history narration in the headscale README. Quick gate green; PR #7573 queued to the merge lane. Memory: none applied.