agents: add Phase 2.3 plan alignment check to worker

- New Phase 2.3 between Phase 2 (Implement) and Phase 2.5 (Verify & Fix)
- Checks implementation scope against epic spec for drift
- Minor drift noted in evidence, major drift escalated via Spec conflict protocol
- Lightweight 30-second gate, not a full re-review

Task: fn-136-borrow-agent-skills-patterns-anti.6

Author: z23cc

agents/worker.md

@@ -413,6 +413,27 @@ The main conversation will resolve the conflict and re-dispatch you (or update t
 - Required API endpoint already exists with different signature
 <!-- /section:core -->
 
+<!-- section:core -->
+## Phase 2.3: Plan Alignment Check
+
+Quick sanity check — did implementation stay within plan scope?
+
+1. Re-read epic spec: `<FLOWCTL> cat <EPIC_ID>`
+2. Compare implementation scope against epic's scope section:
+   - Files changed match expected files in task spec?
+   - No features added beyond what spec described?
+   - No architectural changes not covered in the plan?
+3. If drift detected:
+   - **Minor** (extra helper, renamed file): note in evidence as `"plan_drift": "<description>"`
+   - **Major** (new feature, changed architecture, different approach): send protocol message:
+     ```
+     Spec conflict: <TASK_ID> — implementation diverged from plan.
+     Drift: <description of what changed and why>
+     ```
+
+**This is a 30-second check, not a full re-review.** Read the spec, glance at your diff, note any drift. Then proceed to Phase 2.5.
+<!-- /section:core -->
+
 <!-- section:core -->
 ## Phase 2.5: Verify & Fix