feat: add yaml modeline support for schema definition

[UnyieldingOrca]

This is my attempt at addressing yaml modeline support for check-jsonschema and adding a new precommit hook for running it. This includes support for multi-document yaml processing as well.

Full disclosure this PR was mostly authored with codex + gpt 5.5.

I think this addresses #644 and #340 and would supercede #526.

[sirosen]

Hi there, thanks for working on this! It's something I've wanted to do but haven't been able to make the time for.

I don't have time at present to dig into this and do a careful review, but I have some high-level notes:

• It looks like there's an orthogonal change in here, captured in test_checker_unresolvable_schema_ref_has_concise_error. Could you separate that out into a separate PR? I'm happy to look at changes/improvements to error handling, but mixing it in here will make review harder.
• There also appear to be changes related to reading multiple documents from a single file. Again, I'm very happy to review or work on this, but I don't think it belongs in this PR? Can we remove it for now?
• Thanks for noting model usage! I don't have an official policy on this repo yet, but when I add one it will probably look similar to the one I helped write for pip-tools. So the spirit of that applies -- basically, be honest and try to be good. 😄