fix(worktree): add post-execution cleanup for orphan worktrees

Executor agents spawned with isolation="worktree" committed to temporary
branches in separate working trees, but no step existed to merge those
changes back or clean up. This left orphan worktrees and unmerged
branches after every execution.

Changes:
- execute-phase.md: add step 4.5 "Worktree cleanup" after wave
  completion — merges worktree branch, removes worktree, deletes temp
  branch. Handles merge conflicts gracefully.
- quick.md: add worktree cleanup step after executor returns, before
  summary verification
- Both workflows skip cleanup when workflow.use_worktrees is false
- Both workflows skip silently when no worktrees are found

Closes #1496

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

get-shit-done/workflows/quick.md

@@ -565,9 +565,23 @@ ${AGENT_SKILLS_EXECUTOR}
 ```
 
 After executor returns:
-1. Verify summary exists at `${QUICK_DIR}/${quick_id}-SUMMARY.md`
-2. Extract commit hash from executor output
-3. Report completion status
+1. **Worktree cleanup:** If the executor ran with `isolation="worktree"`, merge the worktree branch back and clean up:
+   ```bash
+   # Find worktrees created by the executor
+   WORKTREES=$(git worktree list --porcelain | grep "^worktree " | grep -v "$(pwd)$" | sed 's/^worktree //')
+   for WT in $WORKTREES; do
+     WT_BRANCH=$(git -C "$WT" rev-parse --abbrev-ref HEAD 2>/dev/null)
+     if [ -n "$WT_BRANCH" ] && [ "$WT_BRANCH" != "HEAD" ]; then
+       git merge "$WT_BRANCH" --no-edit -m "chore: merge quick task worktree ($WT_BRANCH)" 2>&1 || echo "⚠ Merge conflict — resolve manually"
+       git worktree remove "$WT" --force 2>/dev/null || true
+       git branch -D "$WT_BRANCH" 2>/dev/null || true
+     fi
+   done
+   ```
+   If `workflow.use_worktrees` is `false`, skip this step.
+2. Verify summary exists at `${QUICK_DIR}/${quick_id}-SUMMARY.md`
+3. Extract commit hash from executor output
+4. Report completion status
 
 **Known Claude Code bug (classifyHandoffIfNeeded):** If executor reports "failed" with error `classifyHandoffIfNeeded is not defined`, this is a Claude Code runtime bug — not a real failure. Check if summary file exists and git log shows commits. If so, treat as successful.