Validate from stdin

I think it'd be nice to support stdin as an input source. Not sure what the arguments would look like. We could provide `-` similarly to kubectl to indicate files from std in should be validated too?

The issue names no files, tests, or entry points. First clarify the command-line syntax and stdin behavior, including how `-` interacts with file inputs; done means the accepted interface and validation behavior are documented and covered by tests.

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