feat: per-user assistant name and system prompt customization

Each user can rename the AI assistant and edit their own system prompt in Settings.

tests/unit/agent-chat.test.ts

@@ -71,4 +71,16 @@ describe("runAgentChat", () => {
     assert.equal(result.toolCalls[0]?.name, "add_list_item");
     assert.equal(result.toolCalls[0]?.status, 201);
   });
+
+  it("accepts a custom system prompt override", async () => {
+    const result = await runAgentChat({
+      messages: [{ role: "user", content: "hello" }],
+      request: new Request("http://localhost:3000/api/agent/chat"),
+      systemPrompt: "You are a pirate.",
+      llm: createMockLlmClient(),
+    });
+
+    assert.equal(result.message.role, "assistant");
+    assert.ok(result.message.content.length > 0);
+  });
 });