docs: migrate AGENTS.md hub-and-spoke docs to the repo wiki
Wiki is now the documentation home (human- and LLM-friendly): Architecture,
Build-and-Run, Entrypoints, Gotchas, Decisions, Status, Testing, feature
designs (GC-and-Generations, Devshells, Home-Management-Plan), and the
2026-08-21 review record. AGENTS.md in the main repo becomes a brief pointer.
