story-kit: start 258_bug_auto_assign_not_called_after_merge_failure

.story_kit/work/2_current/258_bug_auto_assign_not_called_after_merge_failure.md

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+---
+name: "Auto-assign not called after merge failure"
+---
+
+# Bug 258: Auto-assign not called after merge failure
+
+## Description
+
+When the background merge pipeline fails (e.g. quality gate timeout), `auto_assign_available_work` is never called. The story stays in `4_merge/` with no agent assigned, requiring manual intervention.
+
+### Root cause
+
+In `pool.rs`, `start_merge_agent_work` spawns a tokio task that calls `run_merge_pipeline`. On failure, the task updates the job status to `Failed` but does NOT call `auto_assign_available_work`. The only call to `auto_assign` in the merge pipeline is inside `run_merge_pipeline` on the success path (line ~1251).
+
+The `spawn_pipeline_advance` completion handler does call `auto_assign` after the mergemaster agent exits, but only on the success path (post-merge tests pass → move to done → auto_assign). On failure, it returns early without triggering auto-assign.
+
+There is no periodic sweep — auto-assign is purely reactive (watcher events, agent completions, startup).
+
+### Impact
+
+After a merge failure, the story is permanently stuck in `4_merge/` with no agent. The only way to unstick it is to restart the server or manually trigger a watcher event.
+
+## Acceptance Criteria
+
+- [ ] After a merge pipeline failure, `auto_assign_available_work` is called so the mergemaster can retry
+- [ ] Stories in `4_merge/` do not get permanently stuck after transient merge failures