feat(background-agent): add concurrency group field

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

@@ -28,10 +28,13 @@ export interface BackgroundTask {
   progress?: TaskProgress
   parentModel?: { providerID: string; modelID: string }
   model?: { providerID: string; modelID: string; variant?: string }
-  /** Agent name used for concurrency tracking */
+  /** Active concurrency slot key */
   concurrencyKey?: string
+  /** Persistent key for re-acquiring concurrency on resume */
+  concurrencyGroup?: string
   /** Parent session's agent name for notification */
   parentAgent?: string
+
   /** Last message count for stability detection */
   lastMsgCount?: number
   /** Number of consecutive polls with stable message count */