feat: implement Story 3.1 — chat panel UI with streaming AI responses

Add AI copilot chat panel to the diagram editor with streaming responses,
chat history persistence, and markdown rendering. Includes copilot API route,
diagram-aware system prompt, and schema with 15 passing tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/ai/package.json

@@ -11,7 +11,8 @@
     "./pdf/*": "./src/modules/pdf/*.ts",
     "./tts/*": "./src/modules/tts/*.ts",
     "./stt/*": "./src/modules/stt/*.ts",
-    "./credits/*": "./src/modules/credits/*.ts"
+    "./credits/*": "./src/modules/credits/*.ts",
+    "./copilot/*": "./src/modules/copilot/*.ts"
   },
   "scripts": {
     "clean": "git clean -xdf .cache .turbo dist node_modules",