fix(tmux-subagent): retry pending pane closes to prevent zombie panes

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

src/features/tmux-subagent/polling-manager.test.ts

@@ -12,6 +12,8 @@ describe("TmuxPollingManager overlap", () => {
       description: "test",
       createdAt: new Date(),
       lastSeenAt: new Date(),
+      closePending: false,
+      closeRetryCount: 0,
     })
 
     let activeCalls = 0