Documentation cleanup and centralization via 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 against develop (already scrubbed by Task 11316): checker passes, audit advisories are false positives, all 142 ADRs live with resolving homes, no trackers or stale specs. Nothing to delete; no PR.