fix(engine): skip redundant cleanup ingestion when barrier already ran (fixes #505)

[mickume]

Summary

Eliminates double git ingestion at end-of-run by tracking whether a sync barrier already performed ingestion. The _cleanup_infrastructure function now skips run_background_ingestion when the barrier flag is set, while still running it when no barrier fired (e.g., single-task plans or sync_interval=0).

Closes #505

Changes

File	Change
agent_fox/engine/run.py	Set _barrier_ingestion_ran flag in _barrier_sync; guard cleanup ingestion behind flag check
tests/unit/engine/test_double_ingestion.py	3 regression tests

Tests

• test_cleanup_skips_ingestion_after_barrier — no double ingestion
• test_cleanup_ingests_when_no_barrier_ran — cleanup still works when needed
• test_barrier_failure_still_sets_flag — flag set even on ingestion failure

Verification

• All existing tests pass: ✅ (5246 unchanged)
• New tests pass: ✅ (3 added)
• Linter / formatter: ✅
• No regressions: ✅

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