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/index.ts

@@ -74,7 +74,6 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   const dispose = createPluginDispose({
     backgroundManager: managers.backgroundManager,
     skillMcpManager: managers.skillMcpManager,
-    tmuxSessionManager: managers.tmuxSessionManager,
     disposeHooks: hooks.disposeHooks,
   })