When queryWindowState returned null during session deletion, the
session mapping was deleted but the real tmux pane stayed alive,
creating zombie panes.
- Add closePending/closeRetryCount fields to TrackedSession
- Mark sessions closePending instead of deleting on close failure
- Add retryPendingCloses() called from onSessionCreated and cleanup
- Force-remove mappings after 3 failed retry attempts
- Extract TrackedSessionState helper for field initialization
Tests: 3 pass, 9 expects
YeonGyu-Kim