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/stdio-client.ts

@@ -14,6 +14,7 @@ function getStdioCommand(config: ClaudeCodeMcpServer, serverName: string): strin
 
 export async function createStdioClient(params: SkillMcpClientConnectionParams): Promise<Client> {
   const { state, clientKey, info, config } = params
+  const shutdownGenAtStart = state.shutdownGeneration
 
   const command = getStdioCommand(config, info.serverName)
   const args = config.args ?? []
@@ -55,6 +56,12 @@ export async function createStdioClient(params: SkillMcpClientConnectionParams):
     )
   }
 
+  if (state.shutdownGeneration !== shutdownGenAtStart) {
+    try { await client.close() } catch {}
+    try { await transport.close() } catch {}
+    throw new Error(`MCP server "${info.serverName}" connection completed after shutdown`)
+  }
+
   const managedClient = {
     client,
     transport,