docs(rebrand): R4 — CLAUDE.md mission, roadmap rebrand stage, docs reframe (#5)

- CLAUDE.md: Fork Mission → Project Mission (standalone unofficial, CE-only),
  Upstream Sync → Upstream Cherry-Picks, drop upstream-mergeability constraint
- roadmap: add Stage R (rebrand) with R1-R4 checked off
- plane-api-compat: Cloud column marked historical reference
- docker-release skill: image name plane-ce-mcp

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>