docs: add documentation synchronization workflow to CLAUDE.md
After discovering drift between codebase reality and project tracking
(ATB-10 was complete but marked as Backlog), established a clear
workflow for keeping these synchronized:
- docs/atproto-forum-plan.md (master plan with phase checklist)
- Linear issues (task tracker)
New section explains when and how to update both sources of truth
when completing work, with commit prefix convention for plan updates.
Also updated MEMORY.md with critical reminder about doc sync.