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/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");
+  });
+});