feat: add SyncLevel to sync system for purgatory announcement state-only sync

Purgatory announcements need state events (kind 30618) synced from
external relays, but not full L2/L3 events (patches, issues, PRs)
which would be rejected anyway. This implements the SyncLevel concept
from the design doc (decision #6):

- Add SyncLevel enum (Full vs StateOnly) to RepoSyncNeeds
- When announcement enters purgatory during sync, register in
  RepoSyncIndex with SyncLevel::StateOnly
- Add build_sync_level_aware_filters() that partitions repos by level:
  StateOnly repos only get state event filters (kind 30618)
- Update derive_relay_targets to track state_only_repos separately
- Update compute_actions to handle both repo sets
- SelfSubscriber always uses SyncLevel::Full (promoted repos)

0 files changed, 0 insertions, 0 deletions