The 1353-line advance.rs is split into:
- mod.rs: impl AgentPool with run_pipeline_advance + start_mergemaster_or_block + tests (1244 lines)
- helpers.rs: spawn_pipeline_advance, resolve_qa_mode_from_store, write_review_hold_to_store, should_block_story (128 lines)
Tests stay co-located with run_pipeline_advance which they exercise.
No behaviour change. All 10 advance tests pass; full suite green.
dave