- 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
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