Implements Step 2.2 of the plan:
- EventStore class with append_event, get_event, list_events, is_tombstoned
- Event type constants for all 17 event types from spec
- ClockSkewError for rejecting timestamps >5 min in future
- DuplicateNonceError for idempotency nonce validation
- 25 tests covering all event store functionality
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
