feat: complete isolated-worktree helpers — worktree-review, worktree-clean, tests (#39)

Finishes the isolated-worktree standard begun in #38 (which merged the
branches/ gitignore, runbook, and scripts/worktree-start). Adds the two
remaining helpers and their tests.

- scripts/worktree-review: isolated DETACHED review worktree under
  branches/review-<branch> (fetch/prune first, refuse to overwrite, print path,
  --dry-run). Detached so a reviewer cannot accidentally commit and review work
  never blocks the author's implementation folder.
- scripts/worktree-clean: the only deleting helper — removes a branches/
  worktree after merge/close, refuses a dirty worktree (no --force), optionally
  safe-deletes a merged branch (git branch -d), fetch/prune first, --dry-run.
  Deletes nothing unless explicitly invoked.
- tests/test_worktrees.py: path generation + refuse-to-overwrite for all three
  helpers via --dry-run (no real worktrees/branches/network/deletions).
- runbook: reference worktree-review / worktree-clean and the --dry-run flag.

Checks: bash -n clean on all three scripts; git diff --check clean; full suite
286 passed, 0 failures.

Closes #39. Follow-up to #38.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/llm-workflow-runbooks.md

@@ -205,18 +205,29 @@ git worktree add -b fix/issue-123-example branches/fix-issue-123-example prgs/ma
 cd branches/fix-issue-123-example
 ```
 
-For review work, create a separate review worktree/folder instead of reusing the
-author's implementation folder.
-
-Cleanup is explicit and only after merge or close:
+For review work, create a separate **detached** review worktree instead of
+reusing the author's implementation folder:
 
 ```bash
+scripts/worktree-review fix/issue-123-example   # → branches/review-fix-issue-123-example
+```
+
+Cleanup is explicit and only after merge or close. Use the helper (it fetches/
+prunes first, refuses to remove a dirty worktree, and only safe-deletes a merged
+branch), or the equivalent manual commands:
+
+```bash
+scripts/worktree-clean --delete-branch fix/issue-123-example
+# equivalent manual commands:
 cd <main-repo>
 git fetch prgs --prune
 git worktree remove branches/fix-issue-123-example
 git branch -d fix/issue-123-example
 ```
 
+All three helpers accept `--dry-run` to print the exact commands/paths without
+touching anything.
+
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