fix(#2732): enhance notification for failed/crashed subagent tasks

- completedTaskSummaries now includes status and error info
- notifyParentSession: noReply=false for failed tasks so parent reacts
- Batch notification distinguishes successful vs failed/cancelled tasks
- notification-template updated to show task errors
- task-poller: session-gone tests (85 new lines)
- CI: add Bun shim to PATH for legacy plugin migration tests

src/features/background-agent/manager.ts

@@ -147,7 +147,7 @@ export class BackgroundManager {
   private queuesByKey: Map<string, QueueItem[]> = new Map()
   private processingKeys: Set<string> = new Set()
   private completionTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
-  private completedTaskSummaries: Map<string, Array<{id: string, description: string}>> = new Map()
+  private completedTaskSummaries: Map<string, Array<{id: string, description: string, status: string, error?: string}>> = new Map()
   private idleDeferralTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
   private notificationQueueByParent: Map<string, Promise<void>> = new Map()
   private rootDescendantCounts: Map<string, number>
@@ -1552,6 +1552,8 @@ export class BackgroundManager {
     this.completedTaskSummaries.get(task.parentSessionID)!.push({
       id: task.id,
       description: task.description,
+      status: task.status,
+      error: task.error,
     })
 
     // Update pending tracking and check if all tasks complete
@@ -1573,7 +1575,7 @@ export class BackgroundManager {
     }
 
     const completedTasks = allComplete
-      ? (this.completedTaskSummaries.get(task.parentSessionID) ?? [{ id: task.id, description: task.description }])
+      ? (this.completedTaskSummaries.get(task.parentSessionID) ?? [{ id: task.id, description: task.description, status: task.status, error: task.error }])
       : []
 
     if (allComplete) {
@@ -1591,20 +1593,40 @@ export class BackgroundManager {
 
     let notification: string
     if (allComplete) {
-        const completedTasksText = completedTasks
-          .map(t => `- \`${t.id}\`: ${t.description}`)
-          .join("\n")
+        const succeededTasks = completedTasks.filter(t => t.status === "completed")
+        const failedTasks = completedTasks.filter(t => t.status !== "completed")
+
+        const succeededText = succeededTasks.length > 0
+          ? succeededTasks.map(t => `- \`${t.id}\`: ${t.description}`).join("\n")
+          : ""
+        const failedText = failedTasks.length > 0
+          ? failedTasks.map(t => `- \`${t.id}\`: ${t.description} [${t.status.toUpperCase()}]${t.error ? ` - ${t.error}` : ""}`).join("\n")
+          : ""
+
+        const hasFailures = failedTasks.length > 0
+        const header = hasFailures
+          ? `[ALL BACKGROUND TASKS FINISHED - ${failedTasks.length} FAILED]`
+          : "[ALL BACKGROUND TASKS COMPLETE]"
+
+        let body = ""
+        if (succeededText) {
+          body += `**Completed:**\n${succeededText}\n`
+        }
+        if (failedText) {
+          body += `\n**Failed:**\n${failedText}\n`
+        }
+        if (!body) {
+          body = `- \`${task.id}\`: ${task.description} [${task.status.toUpperCase()}]${task.error ? ` - ${task.error}` : ""}\n`
+        }
 
         notification = `<system-reminder>
-[ALL BACKGROUND TASKS COMPLETE]
+${header}
 
-**Completed:**
-${completedTasksText || `- \`${task.id}\`: ${task.description}`}
+${body.trim()}
 
-Use \`background_output(task_id="<id>")\` to retrieve each result.
+Use \`background_output(task_id="<id>")\` to retrieve each result.${hasFailures ? `\n\n**ACTION REQUIRED:** ${failedTasks.length} task(s) failed. Check errors above and decide whether to retry or proceed.` : ""}
 </system-reminder>`
     } else {
-      // Individual completion - silent notification
       notification = `<system-reminder>
 [BACKGROUND TASK ${statusText}]
 **ID:** \`${task.id}\`
@@ -1612,7 +1634,7 @@ Use \`background_output(task_id="<id>")\` to retrieve each result.
 **Duration:** ${duration}${errorInfo}
 
 **${remainingCount} task${remainingCount === 1 ? "" : "s"} still in progress.** You WILL be notified when ALL complete.
-Do NOT poll - continue productive work.
+${statusText === "COMPLETED" ? "Do NOT poll - continue productive work." : "**ACTION REQUIRED:** This task failed. Check the error and decide whether to retry, cancel remaining tasks, or continue."}
 
 Use \`background_output(task_id="${task.id}")\` to retrieve this result when ready.
 </system-reminder>`
@@ -1675,11 +1697,14 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
           resolvedModel: model,
         })
 
+        const isTaskFailure = task.status === "error" || task.status === "cancelled" || task.status === "interrupt"
+        const shouldReply = allComplete || isTaskFailure
+
         try {
           await this.client.session.promptAsync({
             path: { id: task.parentSessionID },
             body: {
-              noReply: !allComplete,
+              noReply: !shouldReply,
               ...(agent !== undefined ? { agent } : {}),
               ...(model !== undefined ? { model } : {}),
               ...(resolvedTools ? { tools: resolvedTools } : {}),
@@ -1689,7 +1714,8 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
           log("[background-agent] Sent notification to parent session:", {
             taskId: task.id,
             allComplete,
-            noReply: !allComplete,
+            isTaskFailure,
+            noReply: !shouldReply,
           })
         } catch (error) {
           if (isAbortedSessionError(error)) {