fix: address 5 edge cases from review-work findings

- C3: include command args in auto-slash-command dedup key
- H2: track completed task summaries for ALL COMPLETE message
- H9: increment tmux close retry count on re-mark
- H8: detect stale MCP connections after disconnect+reconnect race
- H8: guard disconnectedSessions growth for non-MCP sessions
- C1: await tmux cleanup in plugin dispose lifecycle

src/features/background-agent/manager.ts

@@ -121,6 +121,7 @@ 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 idleDeferralTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
   private notificationQueueByParent: Map<string, Promise<void>> = new Map()
   private rootDescendantCounts: Map<string, number>
@@ -1379,6 +1380,14 @@ export class BackgroundManager {
       })
     }
 
+    if (!this.completedTaskSummaries.has(task.parentSessionID)) {
+      this.completedTaskSummaries.set(task.parentSessionID, [])
+    }
+    this.completedTaskSummaries.get(task.parentSessionID)!.push({
+      id: task.id,
+      description: task.description,
+    })
+
     // Update pending tracking and check if all tasks complete
     const pendingSet = this.pendingByParent.get(task.parentSessionID)
     let allComplete = false
@@ -1398,10 +1407,13 @@ export class BackgroundManager {
     }
 
     const completedTasks = allComplete
-      ? Array.from(this.tasks.values())
-        .filter(t => t.parentSessionID === task.parentSessionID && t.status !== "running" && t.status !== "pending")
+      ? (this.completedTaskSummaries.get(task.parentSessionID) ?? [{ id: task.id, description: task.description }])
       : []
 
+    if (allComplete) {
+      this.completedTaskSummaries.delete(task.parentSessionID)
+    }
+
     const statusText = task.status === "completed"
       ? "COMPLETED"
       : task.status === "interrupt"
@@ -1740,6 +1752,7 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     this.queuesByKey.clear()
     this.processingKeys.clear()
     this.taskHistory.clearAll()
+    this.completedTaskSummaries.clear()
     this.unregisterProcessCleanup()
     log("[background-agent] Shutdown complete")