fix: move state events from Layer 1 to identifier-based filters

Removes kind 30618 (state events) from Layer 1 announcement filter and
adds targeted subscriptions using #d (identifier) tags in Layer 2.

Problem: Layer 1 was receiving ALL state events from all relays,
causing 1000+ rejections for repositories we don't host.

Solution:
- Remove Kind::RepoState from build_announcement_filter (Layer 1)
- Add state_event_filters_for_our_repos() function that creates filters
  with kind 30618 and #d tags for only our hosted repo identifiers
- Integrate state filters into build_layer2_and_layer3_filters
- Extract unique identifiers from repo refs and batch by 100 per filter

Benefits:
- Dramatically reduces bandwidth and rejection noise (1000+ → ~0)
- More efficient: one filter with multiple identifiers vs broadcast
- Only receive state events for repositories we actually care about

Resolves: work/active-issues/layer1-state-event-oversubscription.md

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