feat(ingestion): resolve commit baseline context on sync start (#660)

[aredenba-rh]

Summary

• enrich SyncStarted ingestion handling with Git-backed context preparation before extraction starts
• resolve baseline commit from data-source commit references and pass it into ingestion as checkpoint state
• refresh tracked branch head for GitHub sources and inject tenant-scoped credentials for branch-head resolution/adapter execution

Testing

• uv run pytest tests/unit/ingestion/application/test_ingestion_service.py tests/unit/ingestion/infrastructure/test_ingestion_event_handler.py tests/unit/test_sessioned_ingestion_handler.py -q
• uv run pytest tests/unit/test_application_lifecycle.py tests/unit/ingestion/test_architecture.py -q

Risks

• tracked branch refresh currently targets GitHub sources only (other adapters remain no-op until implemented)

Closes #660

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• Commit unit tests in branch feat/issue-660-ingestion-baseline-resolver

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