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/task-factory.ts

@@ -0,0 +1,18 @@
+import { randomUUID } from "crypto"
+import type { BackgroundTask, LaunchInput } from "../types"
+
+export function createTask(input: LaunchInput): BackgroundTask {
+  return {
+    id: `bg_${randomUUID().slice(0, 8)}`,
+    status: "pending",
+    queuedAt: new Date(),
+    description: input.description,
+    prompt: input.prompt,
+    agent: input.agent,
+    parentSessionID: input.parentSessionID,
+    parentMessageID: input.parentMessageID,
+    parentModel: input.parentModel,
+    parentAgent: input.parentAgent,
+    model: input.model,
+  }
+}