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/cleanup.ts

@@ -95,7 +95,7 @@ export async function disconnectSession(state: SkillMcpManagerState, sessionID:
     }
   }
   if (hasPendingForSession) {
-    state.disconnectedSessions.add(sessionID)
+    state.disconnectedSessions.set(sessionID, (state.disconnectedSessions.get(sessionID) ?? 0) + 1)
   }
   const keysToRemove: string[] = []
 
@@ -125,6 +125,7 @@ export async function disconnectSession(state: SkillMcpManagerState, sessionID:
 }
 
 export async function disconnectAll(state: SkillMcpManagerState): Promise<void> {
+  state.shutdownGeneration++
   stopCleanupTimer(state)
   unregisterProcessCleanup(state)