fix: add toast cleanup to all BackgroundManager task removal paths

TaskToastManager entries were never removed when tasks completed via
error, session deletion, stale pruning, or cancelled with
skipNotification. Ghost entries accumulated indefinitely, causing the
'Queued (N)' count in toast messages to grow without bound.

Added toastManager.removeTask() calls to all 4 missing cleanup paths:
- session.error handler
- session.deleted handler
- cancelTask with skipNotification
- pruneStaleTasksAndNotifications

Closes #1866

src/features/background-agent/manager.ts

@@ -783,6 +783,10 @@ export class BackgroundManager {
       this.cleanupPendingByParent(task)
       this.tasks.delete(task.id)
       this.clearNotificationsForTask(task.id)
+      const toastManager = getTaskToastManager()
+      if (toastManager) {
+        toastManager.removeTask(task.id)
+      }
       if (task.sessionID) {
         subagentSessions.delete(task.sessionID)
       }
@@ -830,6 +834,10 @@ export class BackgroundManager {
         this.cleanupPendingByParent(task)
         this.tasks.delete(task.id)
         this.clearNotificationsForTask(task.id)
+        const toastManager = getTaskToastManager()
+        if (toastManager) {
+          toastManager.removeTask(task.id)
+        }
         if (task.sessionID) {
           subagentSessions.delete(task.sessionID)
         }
@@ -1000,6 +1008,10 @@ export class BackgroundManager {
     }
 
     if (options?.skipNotification) {
+      const toastManager = getTaskToastManager()
+      if (toastManager) {
+        toastManager.removeTask(task.id)
+      }
       log(`[background-agent] Task cancelled via ${source} (notification skipped):`, task.id)
       return true
     }
@@ -1413,6 +1425,10 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
           }
         }
         this.clearNotificationsForTask(taskId)
+        const toastManager = getTaskToastManager()
+        if (toastManager) {
+          toastManager.removeTask(taskId)
+        }
         this.tasks.delete(taskId)
         if (task.sessionID) {
           subagentSessions.delete(task.sessionID)