Add tabcmd/__main__.py so python -m tabcmd works reliably for installed users

[jacalata]

Add tabcmd/main.py so python -m tabcmd works reliably for installed users

Currently, python -m tabcmd works in the repo because of the top-level tabcmd.py, but this relies on file/module resolution rather than the installed package layout.

When tabcmd is installed from PyPI, the repo-root tabcmd.py is not present, so python -m tabcmd will fail unless the package provides tabcmd/main.py.

Adding an explicit main.py makes the behavior unambiguous and aligns with standard Python packaging practices for CLI tools.

Proposed change:

Add a minimal tabcmd/main.py

No behavior change for users invoking tabcmd via the console script
• Improves compatibility with tooling (pipx, PyInstaller, zipapps, etc.)

Optional follow-up (not required for this issue):
• Consider keeping tabcmd.py at repo root as a dev-only shim or removing it entirely to avoid name collisions.