fix: address review-work round 2 findings

- MCP teardown race: add shutdownGeneration counter to prevent
  in-flight connections from resurrecting after disconnectAll
- MCP multi-key disconnect race: replace disconnectedSessions Set
  with generation-based Map to track per-session disconnect events
- MCP clients: check shutdownGeneration in stdio/http client
  creators before inserting into state.clients
- BackgroundManager: call clearTaskHistoryWhenParentTasksGone after
  timer-based task removal in scheduleTaskRemoval and notifyParentSession
- BackgroundManager: clean completedTaskSummaries when parent has
  no remaining tasks
- Plugin dispose: remove duplicate tmuxSessionManager.cleanup call
  since BackgroundManager.shutdown already handles it via onShutdown

src/features/background-agent/manager.ts

@@ -1149,6 +1149,7 @@ export class BackgroundManager {
     if (!parentSessionID) return
     if (this.getTasksByParentSession(parentSessionID).length > 0) return
     this.taskHistory.clearSession(parentSessionID)
+    this.completedTaskSummaries.delete(parentSessionID)
   }
 
   private scheduleTaskRemoval(taskId: string): void {
@@ -1170,6 +1171,7 @@ export class BackgroundManager {
           SessionCategoryRegistry.remove(task.sessionID)
         }
         log("[background-agent] Removed completed task from memory:", taskId)
+        this.clearTaskHistoryWhenParentTasksGone(task?.parentSessionID)
       }
     }, TASK_CLEANUP_DELAY_MS)