docs: update purgatory docs to reflect announcements purgatory implementation

Remove the pre-implementation planning docs (announcements-purgatory-design.md
and announcements-purgatory-implementation.md) now that the feature is built.

Update the three living docs to reflect what was actually implemented:

- purgatory-design.md: expanded to cover all three purgatory stores
  (announcement, state, PR), including AnnouncementPurgatoryEntry structure,
  two-phase soft expiry lifecycle, expiry extension triggers, promotion flow,
  and updated integration points and file structure

- grasp-02-proactive-sync.md: added SyncLevel enum (Full/StateOnly) to
  RepoSyncNeeds, documented the purgatory announcement sync timer as the
  registration path for purgatory announcements, updated filter building
  to describe build_sync_level_aware_filters() and StateOnly behaviour

- grasp-02-proactive-sync-purgatory-git-data.md: expanded to cover
  announcement purgatory as a third entry type, added Timeline E showing
  soft-expiry and revival, replaced the single expiry section with separate
  hard-expiry (state/PR) and two-phase soft-expiry (announcements) sections
  with full justification for the 24-hour extended retention window

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