Add RealSyncContext implementation for production purgatory sync

Implement the production SyncContext that connects to real systems:

- RealSyncContext struct holding purgatory, database, git_data_path,
  our_domain, and local_relay references

- fetch_repository_data: delegates to git::authorization module

- collect_needed_oids: collects commit hashes from state events
  (branches/tags) and PR events (c-tag) in purgatory

- oid_exists: delegates to git::oid_exists function

- fetch_oids: uses git fetch --depth=1 to retrieve specific OIDs
  from remote servers, running in spawn_blocking for async safety

- process_newly_available_git_data: delegates to the unified function
  in git::sync module for consistent post-git-data processing

- has_pending_events: delegates to purgatory method

- find_target_repo: finds first existing owner repository on disk

- our_domain: returns configured domain for clone URL filtering

This enables the purgatory sync loop to use real database queries,
git operations, and event processing instead of mocks.

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