feat(cli): make bot_bottle.cli runnable with python -m

`python -m bot_bottle.cli` failed with "is a package and cannot be
directly executed" — the package had a `if __name__ == "__main__"` block
in `__init__.py`, which never fires for a package and made the invocation
look supported when it wasn't. Add a real `__main__.py` and drop the dead
block.

Matters for `bb login`, whose docstring documents a `bb` entry point that
nothing installs; `-m` is the closest thing to it until there's a
console-script.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/cli/__init__.py

@@ -114,7 +114,3 @@ def main(argv: list[str] | None = None) -> int:
         return e.code if isinstance(e.code, int) else 1
     except KeyboardInterrupt:
         return 130
-
-
-if __name__ == "__main__":
-    sys.exit(main())