fix(background-agent): fix 3 memory leaks in task lifecycle management

H3: cancelTask(skipNotification=true) now schedules task removal.
Previously the early return path skipped cleanup, leaking task objects
in this.tasks Map permanently. Extracted scheduleTaskRemoval() helper
called from both skipNotification and normal paths.

H2: Per-task completion cleanup timer decoupled from allComplete check.
Previously cleanup timer only ran when ALL sibling tasks completed. Now
each finished task gets its own removal timer regardless of siblings.

H1+C2: TaskHistory.clearAll() added and wired into shutdown(). Added
clearSession() calls on session error/deletion and prune cycles.
taskHistory was the only data structure missed by shutdown().

Tests: 10 pass (3 cancel + 3 completion + 4 history)

src/features/background-agent/manager.ts

@@ -906,6 +906,14 @@ export class BackgroundManager {
         this.idleDeferralTimers.delete(task.id)
       }
 
+      this.cleanupPendingByParent(task)
+      this.tasks.delete(task.id)
+      this.clearTaskHistoryWhenParentTasksGone(task.parentSessionID)
+      this.clearNotificationsForTask(task.id)
+      const toastManager = getTaskToastManager()
+      if (toastManager) {
+        toastManager.removeTask(task.id)
+      }
       if (task.sessionID) {
         SessionCategoryRegistry.remove(task.sessionID)
       }
@@ -932,7 +940,12 @@ export class BackgroundManager {
 
       this.pendingNotifications.delete(sessionID)
 
-      if (tasksToCancel.size === 0) return
+      if (tasksToCancel.size === 0) {
+        this.clearTaskHistoryWhenParentTasksGone(sessionID)
+        return
+      }
+
+      const parentSessionsToClear = new Set<string>()
 
       const deletedSessionIDs = new Set<string>([sessionID])
       for (const task of tasksToCancel.values()) {
@@ -942,6 +955,8 @@ export class BackgroundManager {
       }
 
       for (const task of tasksToCancel.values()) {
+        parentSessionsToClear.add(task.parentSessionID)
+
         if (task.status === "running" || task.status === "pending") {
           void this.cancelTask(task.id, {
             source: "session.deleted",
@@ -959,6 +974,10 @@ export class BackgroundManager {
         }
       }
 
+      for (const parentSessionID of parentSessionsToClear) {
+        this.clearTaskHistoryWhenParentTasksGone(parentSessionID)
+      }
+
       this.rootDescendantCounts.delete(sessionID)
       SessionCategoryRegistry.remove(sessionID)
     }
@@ -1125,6 +1144,37 @@ export class BackgroundManager {
     }
   }
 
+  private clearTaskHistoryWhenParentTasksGone(parentSessionID: string | undefined): void {
+    if (!parentSessionID) return
+    if (this.getTasksByParentSession(parentSessionID).length > 0) return
+    this.taskHistory.clearSession(parentSessionID)
+  }
+
+  private scheduleTaskRemoval(taskId: string): void {
+    const existingTimer = this.completionTimers.get(taskId)
+    if (existingTimer) {
+      clearTimeout(existingTimer)
+      this.completionTimers.delete(taskId)
+    }
+
+    const timer = setTimeout(() => {
+      this.completionTimers.delete(taskId)
+      const task = this.tasks.get(taskId)
+      if (task) {
+        this.clearNotificationsForTask(taskId)
+        this.tasks.delete(taskId)
+        this.clearTaskHistoryWhenParentTasksGone(task.parentSessionID)
+        if (task.sessionID) {
+          subagentSessions.delete(task.sessionID)
+          SessionCategoryRegistry.remove(task.sessionID)
+        }
+        log("[background-agent] Removed completed task from memory:", taskId)
+      }
+    }, TASK_CLEANUP_DELAY_MS)
+
+    this.completionTimers.set(taskId, timer)
+  }
+
   async cancelTask(
     taskId: string,
     options?: { source?: string; reason?: string; abortSession?: boolean; skipNotification?: boolean }
@@ -1190,6 +1240,7 @@ export class BackgroundManager {
     removeTaskToastTracking(task.id)
 
     if (options?.skipNotification) {
+      this.scheduleTaskRemoval(task.id)
       log(`[background-agent] Task cancelled via ${source} (notification skipped):`, task.id)
       return true
     }
@@ -1480,29 +1531,8 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
         })
       }
 
-    if (allComplete) {
-      for (const completedTask of completedTasks) {
-        const taskId = completedTask.id
-        const existingTimer = this.completionTimers.get(taskId)
-        if (existingTimer) {
-          clearTimeout(existingTimer)
-          this.completionTimers.delete(taskId)
-        }
-        const timer = setTimeout(() => {
-          this.completionTimers.delete(taskId)
-          const taskToRemove = this.tasks.get(taskId)
-          if (taskToRemove) {
-            this.clearNotificationsForTask(taskId)
-            if (taskToRemove.sessionID) {
-              subagentSessions.delete(taskToRemove.sessionID)
-              SessionCategoryRegistry.remove(taskToRemove.sessionID)
-            }
-            this.tasks.delete(taskId)
-            log("[background-agent] Removed completed task from memory:", taskId)
-          }
-        }, TASK_CLEANUP_DELAY_MS)
-        this.completionTimers.set(taskId, timer)
-      }
+    if (task.status !== "running" && task.status !== "pending") {
+      this.scheduleTaskRemoval(task.id)
     }
   }
 
@@ -1554,6 +1584,7 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
             }
           }
         }
+        this.cleanupPendingByParent(task)
         this.markForNotification(task)
         this.enqueueNotificationForParent(task.parentSessionID, () => this.notifyParentSession(task)).catch(err => {
           log("[background-agent] Error in notifyParentSession for stale-pruned task:", { taskId: task.id, error: err })
@@ -1708,6 +1739,7 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     this.rootDescendantCounts.clear()
     this.queuesByKey.clear()
     this.processingKeys.clear()
+    this.taskHistory.clearAll()
     this.unregisterProcessCleanup()
     log("[background-agent] Shutdown complete")