feat: per-user assistant name and system prompt customization

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

src/lib/assistant-config.ts

@@ -0,0 +1,10 @@
+import { AGENT_SYSTEM_PROMPT } from "@/modules/agent/tools";
+
+export const DEFAULT_ASSISTANT_NAME = "Assistant";
+export const MAX_ASSISTANT_NAME_LENGTH = 40;
+export const MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH = 8000;
+
+export function resolveAssistantSystemPrompt(customPrompt: string | null | undefined): string {
+  const trimmed = customPrompt?.trim();
+  return trimmed ? trimmed : AGENT_SYSTEM_PROMPT;
+}

src/lib/assistant-preference.ts

@@ -1,13 +1,40 @@
 import { eq } from "drizzle-orm";
 import { db } from "@/lib/db";
+import { DEFAULT_ASSISTANT_NAME } from "@/lib/assistant-config";
 import { users } from "@/modules/_core/schema";
 
-export async function getAssistantEnabled(userId: string): Promise<boolean> {
+export {
+  DEFAULT_ASSISTANT_NAME,
+  MAX_ASSISTANT_NAME_LENGTH,
+  MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH,
+  resolveAssistantSystemPrompt,
+} from "@/lib/assistant-config";
+
+export type AssistantPreferences = {
+  enabled: boolean;
+  name: string;
+  systemPrompt: string | null;
+};
+
+export async function getAssistantPreferences(userId: string): Promise<AssistantPreferences> {
   const [row] = await db
-    .select({ assistantEnabled: users.assistantEnabled })
+    .select({
+      assistantEnabled: users.assistantEnabled,
+      assistantName: users.assistantName,
+      assistantSystemPrompt: users.assistantSystemPrompt,
+    })
     .from(users)
     .where(eq(users.id, userId))
     .limit(1);
 
-  return row?.assistantEnabled ?? false;
+  return {
+    enabled: row?.assistantEnabled ?? false,
+    name: row?.assistantName?.trim() || DEFAULT_ASSISTANT_NAME,
+    systemPrompt: row?.assistantSystemPrompt ?? null,
+  };
+}
+
+export async function getAssistantEnabled(userId: string): Promise<boolean> {
+  const prefs = await getAssistantPreferences(userId);
+  return prefs.enabled;
 }