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/tracked-session-state.ts

@@ -0,0 +1,27 @@
+import type { TrackedSession } from "./types"
+
+export function createTrackedSession(params: {
+  sessionId: string
+  paneId: string
+  description: string
+  now?: Date
+}): TrackedSession {
+  const now = params.now ?? new Date()
+
+  return {
+    sessionId: params.sessionId,
+    paneId: params.paneId,
+    description: params.description,
+    createdAt: now,
+    lastSeenAt: now,
+    closePending: false,
+    closeRetryCount: 0,
+  }
+}
+
+export function markTrackedSessionClosePending(tracked: TrackedSession): TrackedSession {
+  return {
+    ...tracked,
+    closePending: true,
+  }
+}