Consider adding --list-variables option to show available template variables

[audreyfeldroy]

Summary

Users automating package creation with key=value arguments need to know which variables are available. Right now they have to either run interactively first or read cookiecutter.json.

Proposal

Add a --list-variables flag that shows available template variables and their defaults:

$ uvx cookiecutter-pypackage --list-variables
Available template variables:
  full_name (default: "Audrey M. Roy Greenfeld")
  email (default: "audreyfeldroy@example.com")
  github_username (default: "audreyfeldroy")
  pypi_package_name (default: "python-boilerplate")
  ...

Implementation considerations

• Would need to read and parse cookiecutter.json
• Could be a separate command instead of a flag: cookiecutter-pypackage show-variables
• Needs to be maintained if template variables change
• Alternatively, could just document the variables in README and link to that

Priority

Low. The interactive flow already shows all variables, and we can document them in the README. This would be nice-to-have for automation.