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/plugin-dispose.ts

@@ -7,12 +7,9 @@ export function createPluginDispose(args: {
   skillMcpManager: {
     disconnectAll: () => Promise<void>
   }
-  tmuxSessionManager: {
-    cleanup: () => Promise<void>
-  }
   disposeHooks: () => void
 }): PluginDispose {
-  const { backgroundManager, skillMcpManager, tmuxSessionManager, disposeHooks } = args
+  const { backgroundManager, skillMcpManager, disposeHooks } = args
   let disposePromise: Promise<void> | null = null
 
   return async (): Promise<void> => {
@@ -23,10 +20,7 @@ export function createPluginDispose(args: {
 
     disposePromise = (async (): Promise<void> => {
       backgroundManager.shutdown()
-      await Promise.all([
-        skillMcpManager.disconnectAll(),
-        tmuxSessionManager.cleanup(),
-      ])
+      await skillMcpManager.disconnectAll()
       disposeHooks()
     })()