The InitStep `script` field docstring claimed only 'sh' or 'ps', but the
step's own VALID_SCRIPT_TYPES = tuple(SCRIPT_TYPE_CHOICES.keys()) is
('sh', 'ps', 'py') and validate() accepts all three (its error message is
built from VALID_SCRIPT_TYPES). Update the docstring to list 'py' too, so
it no longer contradicts the same class's validate() authority.
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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>