storkit: create 428_refactor_split_pool_pipeline_rs_into_submodules

.storkit/work/1_backlog/428_refactor_split_pool_pipeline_rs_into_submodules.md

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+---
+name: "Split pool/pipeline.rs into submodules"
+---
+
+# Refactor 428: Split pool/pipeline.rs into submodules
+
+## Current State
+
+- TBD
+
+## Desired State
+
+Refactor the monolithic server/src/agents/pool/pipeline.rs (1789 lines) into focused submodules.
+
+## Acceptance Criteria
+
+- [ ] advance.rs contains run_pipeline_advance, spawn_pipeline_advance, should_block_story and their unit tests
+- [ ] completion.rs contains run_server_owned_completion, report_completion and their unit tests
+- [ ] merge.rs contains start_merge_agent_work, run_merge_pipeline, get_merge_status, set_merge_failure_reported and their unit tests
+- [ ] mod.rs re-exports all public items and wires the submodules
+- [ ] Unit tests live in their respective module files
+- [ ] No public API changes — all existing imports continue to work
+
+## Out of Scope
+
+- TBD