Provide SKILL.md file for tools like APM to reference

[flcdrg]

I note that there is some documentation on adding a SKILL.md file for Verify in https://github.com/VerifyTests/Verify/blob/main/docs/ai-usage.md

Some projects may like to take advantage of tools like Agent Package Manager. A tool like this can point to SKILL.md files in remote repos and "install" copies of those files into the current repo and do it in an automated way to support numerous agents (eg. Copilot, Claude, etc), as well as propagating updates when the original file is modified.

If Verify had an actual SKILL.md file somewhere in its repo, then I could tell apm to install it from here, which would be nice.