fix(terminal): resolve pendingDelete race and save contention causing crashes

Two bugs in terminal session persistence caused crashes when terminals
were destroyed and recreated with the same ID:

1. pendingDelete blocked legitimate terminal recreation: When a terminal
   exits and is recreated (worktree switch, shell restart), the 5-second
   pendingDelete window silently blocked all session saves for the new
   terminal, leaving it invisible to the session store. Added
   clearPendingDelete() to remove the guard when createTerminal() is
   called with a reused ID.

2. Sync save() and async saveAsync() raced on the same temp file: Both
   methods wrote to terminals.json.tmp without coordination, causing
   ENOENT errors when one renamed a file the other had already moved.
   save() now checks writeInProgress and defers to the async writer
   when a write is in-flight.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

apps/frontend/src/main/terminal-session-store.ts

@@ -195,8 +195,20 @@ export class TerminalSessionStore {
    * 1. Write to temp file
    * 2. Rotate current file to backup
    * 3. Rename temp to target (atomic on most filesystems)
+   *
+   * If an async write is in progress, defers to the async writer to avoid
+   * both operations competing for the same temp file (ENOENT race condition).
    */
   private save(): void {
+    // If an async write is in progress, don't write synchronously — the async
+    // writer shares the same temp file path. Instead, mark a pending write so
+    // saveAsync() will re-save with the latest in-memory data when it finishes.
+    if (this.writeInProgress) {
+      this.writePending = true;
+      debugLog('[TerminalSessionStore] Deferring sync save — async write in progress');
+      return;
+    }
+
     try {
       const content = JSON.stringify(this.data, null, 2);
 
@@ -598,6 +610,27 @@ export class TerminalSessionStore {
     return sessions.find(s => s.id === sessionId);
   }
 
+  /**
+   * Clear a session ID from pendingDelete, allowing saves to proceed.
+   *
+   * Called when a terminal is legitimately re-created with the same ID
+   * (e.g., worktree switching, terminal restart after exit). Without this,
+   * the 5-second pendingDelete window blocks session persistence for the
+   * new terminal.
+   */
+  clearPendingDelete(sessionId: string): void {
+    if (this.pendingDelete.has(sessionId)) {
+      this.pendingDelete.delete(sessionId);
+      // Also clear the cleanup timer since we're explicitly clearing
+      const timer = this.pendingDeleteTimers.get(sessionId);
+      if (timer) {
+        clearTimeout(timer);
+        this.pendingDeleteTimers.delete(sessionId);
+      }
+      debugLog('[TerminalSessionStore] Cleared pendingDelete for re-created terminal:', sessionId);
+    }
+  }
+
   /**
    * Remove a session (from today's sessions)
    *

apps/frontend/src/main/terminal/session-handler.ts

@@ -225,6 +225,18 @@ export function persistAllSessions(terminals: Map<string, TerminalProcess>): voi
   });
 }
 
+/**
+ * Clear a terminal ID from pendingDelete, allowing session saves to proceed.
+ *
+ * Must be called when re-creating a terminal with a previously-used ID
+ * (e.g., worktree switching, terminal restart after shell exit). Without this,
+ * the pendingDelete guard blocks persistence for the new terminal.
+ */
+export function clearPendingDelete(terminalId: string): void {
+  const store = getTerminalSessionStore();
+  store.clearPendingDelete(terminalId);
+}
+
 /**
  * Remove a session from persistent storage
  */

apps/frontend/src/main/terminal/terminal-lifecycle.ts

@@ -54,6 +54,13 @@ export async function createTerminal(
     return { success: true };
   }
 
+  // Clear any pendingDelete for this terminal ID. This handles the case where
+  // a terminal is destroyed and immediately re-created with the same ID (e.g.,
+  // worktree switching, terminal restart after shell exit). Without this, the
+  // pendingDelete guard (5-second window) blocks session persistence for the
+  // new terminal, causing it to be invisible to the session store.
+  SessionHandler.clearPendingDelete(id);
+
   try {
     // For auth terminals, don't inject existing OAuth token - we want a fresh login
     const profileEnv = skipOAuthToken ? {} : PtyManager.getActiveProfileEnv();