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/execute-phase.md

@@ -313,6 +313,39 @@ Execute each selected wave in sequence. Within a wave: parallel if `PARALLELIZAT
    ```
    If hooks fail: report the failure and ask "Fix hook issues now?" or "Continue to next wave?"
 
+4.5. **Worktree cleanup (when `isolation="worktree"` was used):**
+
+   When executor agents ran in worktree isolation, their commits land on temporary branches in separate working trees. After the wave completes, merge these changes back and clean up:
+
+   ```bash
+   # List worktrees created by this wave's agents
+   WORKTREES=$(git worktree list --porcelain | grep "^worktree " | grep -v "$(pwd)$" | sed 's/^worktree //')
+
+   for WT in $WORKTREES; do
+     # Get the branch name for this worktree
+     WT_BRANCH=$(git -C "$WT" rev-parse --abbrev-ref HEAD 2>/dev/null)
+     if [ -n "$WT_BRANCH" ] && [ "$WT_BRANCH" != "HEAD" ]; then
+       CURRENT_BRANCH=$(git rev-parse --abbrev-ref HEAD)
+
+       # Merge the worktree branch into the current branch
+       git merge "$WT_BRANCH" --no-edit -m "chore: merge executor worktree ($WT_BRANCH)" 2>&1 || {
+         echo "⚠ Merge conflict from worktree $WT_BRANCH — resolve manually"
+         continue
+       }
+
+       # Remove the worktree
+       git worktree remove "$WT" --force 2>/dev/null || true
+
+       # Delete the temporary branch
+       git branch -D "$WT_BRANCH" 2>/dev/null || true
+     fi
+   done
+   ```
+
+   **If `workflow.use_worktrees` is `false`:** Agents ran on the main working tree — skip this step entirely.
+
+   **If no worktrees found:** Skip silently — agents may have been spawned without worktree isolation.
+
 5. **Report completion — spot-check claims first:**
 
    For each SUMMARY.md: