refactor: split agents/pool/pipeline/advance.rs into mod.rs + helpers.rs
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.
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