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>

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -41,15 +41,15 @@ story_location: "{project-root}/_bmad-output/implementation-artifacts"
 
 development_status:
   # ── Epic 1: Workspace & Diagram Management (Phase 1 - Foundation) ──
-  epic-1: in-progress
+  epic-1: done
   1-1-create-and-view-diagrams: done
   1-2-organize-diagrams-into-projects: done
   1-3-diagram-access-control-and-management: done
   1-4-recent-view-and-drag-and-drop-organization: done
-  epic-1-retrospective: optional
+  epic-1-retrospective: done
 
   # ── Epic 2: Interactive Canvas & Diagram Types (Phase 2) ──
-  epic-2: in-progress
+  epic-2: done
   2-1-canvas-workspace-with-xyflow-react-and-unified-graph-model: done
   2-2-elk-js-auto-layout-engine-in-web-worker: done
   2-3-bpmn-diagram-type-renderer: done
@@ -59,11 +59,11 @@ development_status:
   2-7-sequence-diagram-type-renderer: done
   2-8-flowchart-diagram-type-renderer: done
   2-9-node-selection-and-manual-repositioning: done
-  epic-2-retrospective: optional
+  epic-2-retrospective: done
 
   # ── Epic 3: AI Copilot & Chat (Phase 2) ──
-  epic-3: backlog
-  3-1-chat-panel-ui-with-streaming-ai-responses: backlog
+  epic-3: in-progress
+  3-1-chat-panel-ui-with-streaming-ai-responses: done
   3-2-ai-diagram-generation-from-natural-language: backlog
   3-3-badge-based-element-referencing-for-targeted-modifications: backlog
   3-4-ai-semantic-suggestions-and-accept-reject-workflow: backlog