Add SyncContext trait and MockSyncContext for purgatory sync

Implement the abstraction layer for purgatory sync operations:

- SyncContext trait: defines interface for repository data fetching,
  OID existence checks, git fetch operations, and event processing
- ProcessResult: captures outcomes when releasing events from purgatory
- MockSyncContext: test mock with builder pattern for configuring:
  - Clone URLs and which OIDs each URL provides
  - Needed OIDs (simulates purgatory state)
  - URL failure simulation
  - Fetch logging for assertions

The trait uses async_trait for async method support and requires
Send + Sync for use in concurrent sync operations.

This abstraction enables unit testing of sync logic without I/O,
while the real implementation (to be added later) will connect
to actual database, git, and relay systems.

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