refactor(background-agent): split manager.ts into focused modules

Extract 30+ single-responsibility modules from manager.ts (1556 LOC):
- task lifecycle: task-starter, task-completer, task-canceller, task-resumer
- task queries: task-queries, task-poller, task-queue-processor
- notifications: notification-builder, notification-tracker, parent-session-notifier
- session handling: session-validator, session-output-validator, session-todo-checker
- spawner: spawner/ directory with focused spawn modules
- utilities: duration-formatter, error-classifier, message-storage-locator
- result handling: result-handler-context, background-task-completer
- shutdown: background-manager-shutdown, process-signal

src/features/background-agent/spawner/spawner-context.ts

@@ -0,0 +1,12 @@
+import type { BackgroundTask } from "../types"
+import type { ConcurrencyManager } from "../concurrency"
+import type { OpencodeClient, OnSubagentSessionCreated } from "../constants"
+
+export interface SpawnerContext {
+  client: OpencodeClient
+  directory: string
+  concurrencyManager: ConcurrencyManager
+  tmuxEnabled: boolean
+  onSubagentSessionCreated?: OnSubagentSessionCreated
+  onTaskError: (task: BackgroundTask, error: Error) => void
+}