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.

package.json

@@ -46,7 +46,12 @@
   },
   "dependencies": {
     "@fontsource/geist": "^5.2.8",
+    "@github/copilot-sdk": "^0.1.25",
     "@hello-pangea/dnd": "^18.0.1",
+    "@langchain/anthropic": "^1.3.19",
+    "@langchain/core": "^1.1.27",
+    "@langchain/langgraph": "^1.1.5",
+    "@langchain/openai": "^1.2.9",
     "@milkdown/crepe": "^7.18.0",
     "@milkdown/kit": "^7.18.0",
     "@tailwindcss/vite": "^4.2.0",