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/task-launch.ts

@@ -0,0 +1,77 @@
+import { getTaskToastManager } from "../task-toast-manager"
+import { log } from "../../shared"
+
+import type { BackgroundTask } from "./types"
+import type { LaunchInput } from "./types"
+
+type QueueItem = {
+  task: BackgroundTask
+  input: LaunchInput
+}
+
+export function launchBackgroundTask(args: {
+  input: LaunchInput
+  tasks: Map<string, BackgroundTask>
+  pendingByParent: Map<string, Set<string>>
+  queuesByKey: Map<string, QueueItem[]>
+  getConcurrencyKeyFromInput: (input: LaunchInput) => string
+  processKey: (key: string) => void
+}): BackgroundTask {
+  const { input, tasks, pendingByParent, queuesByKey, getConcurrencyKeyFromInput, processKey } = args
+
+  log("[background-agent] launch() called with:", {
+    agent: input.agent,
+    model: input.model,
+    description: input.description,
+    parentSessionID: input.parentSessionID,
+  })
+
+  if (!input.agent || input.agent.trim() === "") {
+    throw new Error("Agent parameter is required")
+  }
+
+  const task: BackgroundTask = {
+    id: `bg_${crypto.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,
+    category: input.category,
+  }
+
+  tasks.set(task.id, task)
+
+  if (input.parentSessionID) {
+    const pending = pendingByParent.get(input.parentSessionID) ?? new Set<string>()
+    pending.add(task.id)
+    pendingByParent.set(input.parentSessionID, pending)
+  }
+
+  const key = getConcurrencyKeyFromInput(input)
+  const queue = queuesByKey.get(key) ?? []
+  queue.push({ task, input })
+  queuesByKey.set(key, queue)
+
+  log("[background-agent] Task queued:", { taskId: task.id, key, queueLength: queue.length })
+
+  const toastManager = getTaskToastManager()
+  if (toastManager) {
+    toastManager.addTask({
+      id: task.id,
+      description: input.description,
+      agent: input.agent,
+      isBackground: true,
+      status: "queued",
+      skills: input.skills,
+    })
+  }
+
+  processKey(key)
+  return task
+}