feat(cli): add --delete flag to /exit command for session deletion (#19332)

Co-authored-by: David Pierce <davidapierce@google.com>

docs/reference/commands.md

@@ -323,6 +323,11 @@ Slash commands provide meta-level control over the CLI itself.
 ### `/quit` (or `/exit`)
 
 - **Description:** Exit Gemini CLI.
+- **Flags:**
+  - **`--delete`** _(optional)_: Exit and permanently delete the current
+    session's history and temporary files (chat recording, tool outputs). Useful
+    for privacy or one-off tasks where you don't want to leave any traces.
+  - **Usage:** `/quit --delete` or `/exit --delete`
 
 ### `/restore`