refactor(delegate-task): improve session title format and add task_metadata block

- Change session title from 'Task: {desc}' to '{desc} (@{agent} subagent)'
- Move session_id to structured <task_metadata> block for better parsing
- Add category tracking to BackgroundTask type and LaunchInput
- Add tests for new title format and metadata block

src/features/background-agent/manager.ts

@@ -138,6 +138,7 @@ export class BackgroundManager {
       parentModel: input.parentModel,
       parentAgent: input.parentAgent,
       model: input.model,
+      category: input.category,
     }
 
     this.tasks.set(task.id, task)
@@ -231,7 +232,7 @@ export class BackgroundManager {
     const createResult = await this.client.session.create({
       body: {
         parentID: input.parentSessionID,
-        title: `Background: ${input.description}`,
+        title: `${input.description} (@${input.agent} subagent)`,
         permission: [
           { permission: "question", action: "deny" as const, pattern: "*" },
         ],

src/features/background-agent/types.ts

@@ -38,6 +38,8 @@ export interface BackgroundTask {
   parentAgent?: string
   /** Marks if the task was launched from an unstable agent/category */
   isUnstableAgent?: boolean
+  /** Category used for this task (e.g., 'quick', 'visual-engineering') */
+  category?: string
 
   /** Last message count for stability detection */
   lastMsgCount?: number
@@ -57,6 +59,7 @@ export interface LaunchInput {
   isUnstableAgent?: boolean
   skills?: string[]
   skillContent?: string
+  category?: string
 }
 
 export interface ResumeInput {