feat(cli): add upgrade alias and register mail command

- Add upgrade as an alias for update
- Register mail_cmd in CLI entrypoint

myclaude/commands/__init__.py

@@ -1,6 +1,10 @@
 """CLI subcommands."""
 
 from .chat import chat_cmd
+from .mail import mail_cmd
 from .update import update_cmd
 
-__all__ = ["chat_cmd", "update_cmd"]
+# alias: upgrade is the same as update
+upgrade_cmd = update_cmd
+
+__all__ = ["chat_cmd", "mail_cmd", "update_cmd", "upgrade_cmd"]