feat: replace Claude with GPT for AI chat integration

- Replace ClaudeAdapter with GPTAdapter using OpenAI GPT (gpt-5-mini)
- Implement function calling for calendar operations (getDay, proposeCreate/Update/Delete, searchEvents)
- Add provider-agnostic AI utilities in ai/utils/ (systemPrompt, toolDefinitions, toolExecutor, eventFormatter)
- Add USE_TEST_RESPONSES env var to toggle between real AI and test responses
- Switch ChatService.processMessage to use real AI provider
- Add npm run format command for Prettier
- Update CLAUDE.md with new architecture

apps/server/src/services/ChatService.ts

@@ -265,12 +265,26 @@ export class ChatService {
       content: data.content,
     });
 
-    const response = await getTestResponse(
-      responseIndex,
-      this.eventRepo,
-      userId,
-    );
-    responseIndex++;
+    let response: TestResponse;
+
+    if (process.env.USE_TEST_RESPONSES === "true") {
+      // Test mode: use static responses
+      response = await getTestResponse(responseIndex, this.eventRepo, userId);
+      responseIndex++;
+    } else {
+      // Production mode: use real AI
+      const events = await this.eventRepo.findByUserId(userId);
+      const history = await this.chatRepo.getMessages(conversationId, {
+        limit: 20,
+      });
+
+      response = await this.aiProvider.processMessage(data.content, {
+        userId,
+        conversationHistory: history,
+        existingEvents: events,
+        currentDate: new Date(),
+      });
+    }
 
     // Save and then return assistant response
     const answerMessage = await this.chatRepo.createMessage(conversationId, {
@@ -292,7 +306,9 @@ export class ChatService {
     updates?: UpdateEventDTO,
   ): Promise<ChatResponse> {
     // Update original message with respondedAction
-    await this.chatRepo.updateMessage(messageId, { respondedAction: "confirm" });
+    await this.chatRepo.updateMessage(messageId, {
+      respondedAction: "confirm",
+    });
 
     // Perform the actual event operation
     let content: string;