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/db/migrations/meta/_journal.json

@@ -15,6 +15,13 @@
       "when": 1771885601062,
       "tag": "0001_fuzzy_gorilla_man",
       "breakpoints": true
+    },
+    {
+      "idx": 2,
+      "version": "7",
+      "when": 1772245471347,
+      "tag": "0002_numerous_siren",
+      "breakpoints": true
     }
   ]
 }