persist and restore announcement events across graceful restarts

Extends purgatory persistence to include announcement purgatory entries.
On graceful shutdown, non-soft-expired announcements are serialised to
purgatory-state.json alongside state/PR/expired events; on startup they
are restored, skipping any entry whose bare repo path no longer exists.

Updates purgatory-design.md to reflect that purgatory persists through
graceful shutdown and documents the new PurgatoryState disk format.

Adds create_announcement_event helper to purgatory_helpers and three new
integration tests in purgatory_persistence covering the full save/restore
cycle, missing-repo skip, and the combined roundtrip with all entry types.

0 files changed, 0 insertions, 0 deletions