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/__main__.py

@@ -0,0 +1,15 @@
+"""Entry point for `python -m bot_bottle.cli`.
+
+`cli.py` at the repo root is the usual way in; this makes the package
+runnable too, so the CLI works from an installed copy where there is no
+`cli.py` on disk to point at.
+"""
+
+from __future__ import annotations
+
+import sys
+
+from . import main
+
+if __name__ == "__main__":
+    sys.exit(main())