feat(background-agent): track spawn depth on tasks

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/types.ts

@@ -19,11 +19,13 @@ export interface TaskProgress {
 export interface BackgroundTask {
   id: string
   sessionID?: string
+  rootSessionID?: string
   parentSessionID: string
   parentMessageID: string
   description: string
   prompt: string
   agent: string
+  spawnDepth?: number
   status: BackgroundTaskStatus
   queuedAt?: Date
   startedAt?: Date