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/skill-mcp-manager/manager.ts

@@ -10,12 +10,13 @@ export class SkillMcpManager {
   private readonly state: SkillMcpManagerState = {
     clients: new Map(),
     pendingConnections: new Map(),
-    disconnectedSessions: new Set(),
+    disconnectedSessions: new Map(),
     authProviders: new Map(),
     cleanupRegistered: false,
     cleanupInterval: null,
     cleanupHandlers: [],
     idleTimeoutMs: 5 * 60 * 1000,
+    shutdownGeneration: 0,
   }
 
   private getClientKey(info: SkillMcpClientInfo): string {