story-kit: create 121_story_test_coverage_io_watcher_rs

.story_kit/work/1_upcoming/121_story_test_coverage_io_watcher_rs.md

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+---
+name: "Add test coverage for io/watcher.rs (40% -> 70%+)"
+---
+
+# Story 121: Add test coverage for io/watcher.rs
+
+Currently at 40% line coverage (238 lines, 142 missed). The file watcher is critical infrastructure — it drives pipeline advancement and auto-commits.
+
+## What to test
+
+- Story file detection and classification (which directory, what kind of move)
+- Debounce/flush logic
+- Git add/commit message generation
+- Watcher pause/resume mechanism (added in story 119 for merge safety)
+- Edge cases: rapid file changes, missing directories, git failures
+
+## Notes
+
+- Use temp directories for filesystem tests
+- Mock git commands where needed
+- The watcher pause lock is especially important to test given its role in merge safety
+
+## Acceptance Criteria
+
+- [ ] Line coverage for `io/watcher.rs` reaches 70%+
+- [ ] Tests pass with `cargo test`
+- [ ] `cargo clippy` clean