# Implementation: add-blast-radius-scheduler

Implements per-loop git worktree creation in `scripts/loop-runner.py` per SPEC R1-R6.

## Files changed

- `scripts/loop-runner.py` -- added `_git_run`, `_ensure_worktree`, `LOOP_WORKTREE_DIR` constant; replaced step 4 stub with worktree creation; updated module docstring.
- `tests/test_blast_radius.py` -- 15 tests covering R1-R6 + regression.

## R-by-R coverage

| Req | Code |
|-----|------|
| R1 _ensure_worktree | `_ensure_worktree(state, cfg, loop_path, project_dir)` -- checks `blast_radius.use_worktree` (default True), reuses existing worktree, creates new via `git worktree add` |
| R2 graceful degradation | `_git_run` catches `OSError`/`SubprocessError`; not-a-repo and worktree-add failures log WARNING and return `project_dir` |
| R3 cmd_tick integration | Step 4 replaced: `cwd = _ensure_worktree(state, cfg, loop_path, project_dir)` |
| R4 branch already exists | First try `-b loop/<name>`; on "already exists" in stderr, retry without `-b` (checkout existing branch) |
| R5 state consistency | Stale `worktree_path` (path doesn't exist) is cleared before recreation; state written atomically via `_write_state_loop` |
| R6 platform paths | `pathlib.Path` for all path construction; git handles OS-specific normalization |
| R7 doc updates | technical.md section 7 step 4 updated; CHANGELOG.md updated |
| R8 tests | 15 tests in `tests/test_blast_radius.py` |

## Key design decisions

- `use_worktree` defaults to `True` when the field is missing (D2: "default is worktree-on").
- Worktree path is `<loop_path>/worktree` (matches `LOOP_WORKTREE_DIR` in status.py).
- Branch name is `loop/<loop_name>` (matches technical.md section 7 step 4).
- `_git_run` is a thin wrapper around `subprocess.run(["git", ...])` that returns `(rc, stdout, stderr)` and catches all `OSError`/`SubprocessError`.
- State is written inside `_ensure_worktree` (not deferred to step 10) because the worktree exists on disk immediately after creation; recording it in state is correct even if the tick crashes later.
- The drift gate (`_gate_worktree_drift` in status.py) already handles the case where `worktree_path` is null (skips the check). So fallback to project root is safe.

## Tests (`tests/test_blast_radius.py`)

15 tests across 6 classes; all `subprocess.run` calls stubbed via monkeypatch.

- `TestEnsureWorktree` (4): creates worktree; reuses existing; use_worktree=false returns project root; missing field defaults true.
- `TestGracefulDegradation` (3): falls back when not git repo; falls back when git missing; falls back when worktree add fails.
- `TestBranchExists` (1): reuses existing branch (retry without -b).
- `TestStateConsistency` (2): clears stale worktree path; recreates after deletion.
- `TestTickIntegration` (3): first tick creates worktree; second tick reuses; falls back when no git.
- `TestPlatformPaths` (1): worktree path constructed via pathlib.
- `TestRegression` (1): existing loop with worktree_path ticks unchanged.

## Verification

- `python3 -m py_compile scripts/loop-runner.py` -- PASS
- `python3 -m pytest tests/test_blast_radius.py -v` -- 15 passed
- `python3 -m pytest tests/ -q` -- 369 passed (354 + 15 new)
- `bash -n scripts/*.sh` -- no shell changes