story-kit: create 128_story_test_coverage_worktree_rs

.story_kit/work/1_upcoming/128_story_test_coverage_worktree_rs.md

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+---
+name: "Add test coverage for worktree.rs (65% -> 80%+)"
+---
+
+# Story 128: Add test coverage for worktree.rs
+
+Currently at 65% line coverage (330 lines, 124 missed). Worktree management is core infrastructure — creating, removing, and managing git worktrees for agent isolation.
+
+## What to test
+
+- `worktree_path` construction
+- `create_worktree` — branch naming, git worktree add, setup command execution
+- `remove_worktree_by_story_id` — cleanup, branch deletion
+- Setup command runner (pnpm install, pnpm build, cargo check)
+- Error paths: git failures, setup failures, missing directories
+- Edge cases: worktree already exists, branch already exists
+
+## Notes
+
+- Use temp git repos for integration tests
+- Mock expensive operations (pnpm install, cargo check) where possible
+- The setup command failure path is especially important (this was the root cause of bug 118)
+
+## Acceptance Criteria
+
+- [ ] Line coverage for `worktree.rs` reaches 80%+
+- [ ] Tests pass with `cargo test`
+- [ ] `cargo clippy` clean