feat: add ask_chatgpt tool handler

TASKS.md

@@ -299,6 +299,21 @@ All six prompt builders are implemented and tested:
 - No OpenAI API calls from the builders.
 - No file loading, logging, or context budget within the builders.
 
+### Task 4.1 - ask_chatgpt MCP tool handler
+
+Create `src/tools/ask-chatgpt.js` and `test/tools/ask-chatgpt.test.js`.
+
+**Requirements:**
+- Export `handleAskChatGpt(input, deps)` as standalone dependency-injected function.
+- Execution order: validateToolInput → loadConfig → checkContextBudget → buildAskChatGptPrompt → createOpenAIClient → sendOpenAIResponse.
+- Inject only 3 external deps: `loadConfig`, `createOpenAIClient`, `sendOpenAIResponse`. Internal utilities imported directly.
+- If budget check fails (`ok: false`), immediately return that result — do not call buildAskChatGptPrompt or create client.
+- All paths return structured `{ ok, answer|error, warnings }` — never throws to caller.
+- OpenAI errors pass through `String(err)` unchanged (no wrapping/reformatting).
+- 27 orchestration-only tests covering: success path, validation failure, config failure, budget failure, client creation failure, OpenAI failure, dependency call order, error handling for null/string throws, warning propagation, result shape, and short-circuit behavior.
+
+Status: ✅ Complete
+
 ---
 
 Phase 2 complete. Phase 3 complete. Phase 4 next: Tool Handlers.