feat(FN-1181): add ReflectionStore with agent reflection models
- Add reflection core types for triggers, metrics, persisted reflections, and aggregated performance summaries
- Implement ReflectionStore with JSONL persistence, per-agent write locking, retrieval/latest helpers, summary aggregation, and cleanup APIs
- Emit typed reflection events for creation and summary computation and export the new store/types from @fusion/core
- Add comprehensive ReflectionStore tests covering persistence order, malformed JSONL handling, windowed summaries, events, deletion, and concurrent writes
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