Part of #3277 (foundation step).

Why: Everything else builds on the CLI knowing about a Python variant and being able to invoke it portably.

Tasks

• Add "py": "Python" to SCRIPT_TYPE_CHOICES in src/specify_cli/_agent_config.py.
• Verify process_template() (src/specify_cli/integrations/base.py) extracts py: from frontmatter via the existing generic script_type logic (should need no change — confirm).
• Update install_scripts() to copy .py files (and handle the .sh-only chmod path appropriately).
• Add an interpreter-resolution helper (prefer project venv → python3 → python) for the {SCRIPT} invocation string.
• Update VALID_SCRIPT_TYPES consumers: src/specify_cli/workflows/steps/init/, src/specify_cli/integrations/_helpers.py, src/specify_cli/commands/init.py.

Acceptance

• specify init x --script py selects the Python variant without error.
• Unit tests cover the new choice.