feat: add architecture_review tool handler

TASKS.md

@@ -372,8 +372,37 @@ Create `src/tools/architecture-review.js` and `test/tools/architecture-review.te
 - OpenAI errors pass through `String(err)` unchanged (no wrapping/reformatting).
 - Mirror 4.1/4.2/4.3/4.4 structure: ~28 orchestration-only tests covering the same test categories.
 
-Status: ⬜ Pending
+Status: ✅ Complete
+
+### Task 4.6 — Phase 4 completion summary
+
+**Phase 4 — Tool Handlers** is now complete.
+
+All five tool handlers are implemented and tested:
+
+| # | Handler | File | Tests |
+|---|---------|------|-------|
+| 1 | `handleAskChatGpt` | `src/tools/ask-chatgpt.js` | ✅ 27 |
+| 2 | `handleReviewPlan` | `src/tools/review-plan.js` | ✅ 28 |
+| 3 | `handleReviewCode` | `src/tools/review-code.js` | ✅ 28 |
+| 4 | `handleDebugIssue` | `src/tools/debug-issue.js` | ✅ 28 |
+| 5 | `handleArchitectureReview` | `src/tools/architecture-review.js` | ✅ 28 |
+
+**What Phase 4 established:**
+
+- Five standalone, dependency-injected handlers following the same orchestration pattern: validate → config → budget → prompt → client → response.
+- All external deps injected (loadConfig, createOpenAIClient, sendOpenAIResponse); internal utilities imported directly.
+- Every handler returns structured `{ ok, answer|error, warnings }` — never throws to caller.
+- Budget check short-circuits before prompt building or client creation.
+- OpenAI errors pass through `String(err)` unchanged — no wrapping or reformatting.
+- 139 orchestration-only tests covering success, validation failure, config failure, budget failure, client failure, OpenAI failure, call order, prompt integration, throws escaping, warnings, result shape, and short-circuit behavior.
+
+**What Phase 4 did NOT do:**
+
+- No MCP tool registration yet.
+- No server or router code yet.
+- That belongs to Phase 5.
 
 ---
 
-Phase 2 complete. Phase 3 complete. Phase 4 next: Tool Handlers.
+Phase 2 complete. Phase 3 complete. Phase 4 complete. Phase 5 next: MCP Server and Tool Registration.