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/result-handler-context.ts

@@ -0,0 +1,9 @@
+import type { OpencodeClient } from "./constants"
+import type { ConcurrencyManager } from "./concurrency"
+import type { TaskStateManager } from "./state"
+
+export interface ResultHandlerContext {
+  client: OpencodeClient
+  concurrencyManager: ConcurrencyManager
+  state: TaskStateManager
+}