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/e2e/assistant.spec.ts

@@ -16,7 +16,7 @@ test("assistant chat smoke after opt-in", async ({ page }) => {
   await page.getByRole("button", { name: "Open assistant" }).click();
   await expect(page.getByRole("dialog", { name: "Assistant" })).toBeVisible();
 
-  await page.getByLabel("Assistant message").fill("hello assistant");
+  await page.getByLabel("Message for Assistant").fill("hello assistant");
   await page.getByRole("button", { name: "Send" }).click();
 
   await expect(page.getByText("hello assistant")).toBeVisible();

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);
+  });
 });

tests/unit/assistant-preference.test.ts

@@ -0,0 +1,27 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  DEFAULT_ASSISTANT_NAME,
+  resolveAssistantSystemPrompt,
+} from "../../src/lib/assistant-config";
+import { AGENT_SYSTEM_PROMPT } from "../../src/modules/agent/tools";
+
+describe("resolveAssistantSystemPrompt", () => {
+  it("returns the default prompt when custom is null", () => {
+    assert.equal(resolveAssistantSystemPrompt(null), AGENT_SYSTEM_PROMPT);
+  });
+
+  it("returns the default prompt when custom is blank", () => {
+    assert.equal(resolveAssistantSystemPrompt("   "), AGENT_SYSTEM_PROMPT);
+  });
+
+  it("returns trimmed custom prompt when set", () => {
+    assert.equal(resolveAssistantSystemPrompt("  Be extra cheerful.  "), "Be extra cheerful.");
+  });
+});
+
+describe("DEFAULT_ASSISTANT_NAME", () => {
+  it("is Assistant", () => {
+    assert.equal(DEFAULT_ASSISTANT_NAME, "Assistant");
+  });
+});