feat: add review_plan tool handler

AGENT_HANDOFF.md

@@ -40,6 +40,16 @@ Build the MCP tool handlers that:
 ## Completed (Phase 4)
 
 - Task 4.1 — ask_chatgpt handler (`src/tools/ask-chatgpt.js`, `test/tools/ask-chatgpt.test.js`) ✅
+- Task 4.2 — review_plan handler (`src/tools/review-plan.js`, `test/tools/review-plan.test.js`) ✅
+
+## Next Pending
+
+### Task 4.3 - review_code tool handler
+
+Build the review_code MCP tool handler:
+1. Create `src/tools/review-code.js` with dependency-injected `handleReviewCode(input, deps)`
+2. Create `test/tools/review-code.test.js` with ~28 orchestration-only tests
+3. Follow the same pattern as 4.1 and 4.2
 
 ## General Rules