Add GitHub Actions workflow command reporter

[kehoecj]

Description

Add a --reporter=github option that outputs validation errors as GitHub Actions workflow commands. This enables inline PR annotations without needing to use the separate validate configs action

Proposed behavior

validator --reporter=github .

Output format:

::error file=config/bad.json,line=3,col=12::syntax: unexpected token
::error file=deploy/broken.yaml,line=15,col=4::syntax: mapping values are not allowed here

Valid files produce no output (workflow commands are errors/warnings only).

Why

• The validate-configs-action already provides PR annotations, but it requires using the GitHub Action wrapper
• Many users run the validator CLI directly in their workflow steps — --reporter=github would give them annotations for free
• Mimics how tools like golangci-lint, eslint, and shellcheck integrate with GitHub Actions