feat: Integrate GitHub Copilot SDK and LangChain for provider-independent orchestration

- Added @github/copilot-sdk and related dependencies for GitHub Copilot integration.
- Implemented ChatCopilot adapter for LangChain compatibility.
- Created LLM factory to return provider-specific models (OpenAI, Anthropic, Copilot).
- Developed Orchestrator agent using LangGraph for intent routing and context assembly.
- Enhanced IPC communication for streaming AI responses to the renderer.
- Updated progress documentation with implementation details and learnings.

src/main/ai/copilot.ts

@@ -1,5 +1,10 @@
+import { app } from 'electron';
 import { registerProvider, type AIProvider } from './provider';
 
+// Dynamic import type — @github/copilot-sdk is ESM-only
+type CopilotClientType = import('@github/copilot-sdk').CopilotClient;
+
+let client: CopilotClientType | null = null;
 let token: string | null = null;
 
 const copilotProvider: AIProvider = {
@@ -8,13 +13,33 @@ const copilotProvider: AIProvider = {
 
   async initialize(t: string): Promise<boolean> {
     token = t;
-    // Actual GitHub Copilot SDK client creation will be added in US-019.
-    // For now, having a token means the provider is ready.
-    return true;
+    try {
+      // Stop existing client if re-initializing
+      if (client) {
+        // eslint-disable-next-line @typescript-eslint/no-empty-function
+        await client.stop().catch(() => {});
+        client = null;
+      }
+
+      const { CopilotClient } = await import('@github/copilot-sdk');
+      client = new CopilotClient({
+        githubToken: t,
+        autoStart: true,
+        autoRestart: true,
+        logLevel: 'warning',
+      });
+      await client.start();
+      console.log('[AI] CopilotClient started successfully');
+      return true;
+    } catch (err) {
+      console.error('[AI] Failed to start CopilotClient:', err);
+      client = null;
+      return false;
+    }
   },
 
   isReady(): boolean {
-    return token !== null;
+    return client !== null && token !== null;
   },
 };
 
@@ -23,4 +48,16 @@ export function getCopilotToken(): string | null {
   return token;
 }
 
+/** Get the CopilotClient instance (null if not initialized). */
+export function getCopilotClient(): CopilotClientType | null {
+  return client;
+}
+
+// Clean shutdown on app quit
+app.on('before-quit', () => {
+  if (client) {
+    client.stop().catch((err: unknown) => console.error('[AI] Error stopping CopilotClient:', err));
+  }
+});
+
 registerProvider(copilotProvider);