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/modules/agent/server/run.ts

@@ -26,6 +26,7 @@ export type AgentProgressHandler = (event: AgentProgressEvent) => void;
 export async function runAgentChat(options: {
   messages: ClientChatMessage[];
   request: Request;
+  systemPrompt?: string;
   llm?: LlmClient;
   executeTool?: ToolExecutor;
   onProgress?: AgentProgressHandler;
@@ -33,9 +34,10 @@ export async function runAgentChat(options: {
   const llm = options.llm ?? createLlmClient();
   const executeTool = options.executeTool ?? createApiToolExecutor(options.request);
   const onProgress = options.onProgress;
+  const systemPrompt = options.systemPrompt ?? AGENT_SYSTEM_PROMPT;
 
   const transcript: ChatMessage[] = [
-    { role: "system", content: AGENT_SYSTEM_PROMPT },
+    { role: "system", content: systemPrompt },
     ...options.messages.map(
       (message): ChatMessage => ({
         role: message.role,