Refresh docs after the editor/dashboard/landing rework
- README: project layout now matches src/ (feed·seo·editor·landing·lexicon
libs, HandleStart/ErrorScene components, lexicon/404/500/rss pages); How it
works covers publication pages, full-content RSS, OG/Twitter cards, dashboard.
- SP9: mark the lexicon page Built; entry point is the footer Docs link, not
the retired home-page hero button.
- lexicons/README: updatedAt reflects the live edit flow, not 'reserved (SP5)'.
- AGENTS: clarify SP0-SP12 are the v1 build archive; post-v1 work lives in
dated docs/superpowers/ design+plan pairs.