Add `py:` lines to command templates' `scripts:` frontmatter

[mnriem]

Part of #3277. Depends on the core + extension ports.

Tasks

• Add a py: line to the scripts: block in every templates/commands/*.md (and extension templates).
• Verify {SCRIPT} substitution renders correct Python invocations for the py script type.

Acceptance

• Rendering a command with --script py produces a valid Python invocation string.

[marcelsafin]

I'd like to take this one, as a follow-up to the ports in #3386/#3387/#3400. Plan: add py: lines to the scripts: frontmatter in all command templates (core + extensions), pointing at the Python twins, and extend the rendering tests to assert --script py produces valid invocation strings.