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/src/modules/copilot/system-prompt.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from "vitest";
+
+import { buildCopilotSystemPrompt } from "./system-prompt";
+
+import type { DiagramType } from "./types";
+
+describe("buildCopilotSystemPrompt", () => {
+  it("should include the diagram type in uppercase", () => {
+    const prompt = buildCopilotSystemPrompt("bpmn");
+    expect(prompt).toContain("BPMN");
+    expect(prompt).toContain("Type: BPMN");
+  });
+
+  it("should include diagram description for each type", () => {
+    const types: DiagramType[] = [
+      "bpmn",
+      "er",
+      "orgchart",
+      "architecture",
+      "sequence",
+      "flowchart",
+    ];
+
+    for (const type of types) {
+      const prompt = buildCopilotSystemPrompt(type);
+      expect(prompt).toContain(`Type: ${type.toUpperCase()}`);
+      expect(prompt).toContain("domaingraph AI copilot");
+      expect(prompt).toContain("CHAT-ONLY mode");
+    }
+  });
+
+  it("should mention chat-only constraint", () => {
+    const prompt = buildCopilotSystemPrompt("er");
+    expect(prompt).toContain("CHAT-ONLY mode");
+    expect(prompt).toContain("cannot modify the diagram directly");
+  });
+
+  it("should include today's date", () => {
+    const prompt = buildCopilotSystemPrompt("flowchart");
+    const year = new Date().getFullYear().toString();
+    expect(prompt).toContain(year);
+  });
+
+  it("should include ER-specific description", () => {
+    const prompt = buildCopilotSystemPrompt("er");
+    expect(prompt).toContain("Entity-Relationship");
+    expect(prompt).toContain("entities");
+  });
+
+  it("should include architecture-specific description", () => {
+    const prompt = buildCopilotSystemPrompt("architecture");
+    expect(prompt).toContain("services");
+    expect(prompt).toContain("databases");
+  });
+});