Add JSON schema validation to CI

[Will-thom]

Summary

• validate generated demo JSON reports against schema/prmaven-report.schema.json with Go tests
• add a dependency-light CI job for JSON schema validation
• document the focused schema validation command in testing and CI docs
• emit empty findings as [] so no-failure JSON matches the schema contract

Validation

• go test ./pkg/prmaven -run TestGeneratedJSONReportsValidateAgainstSchema -v
• git diff --check
• .github/scripts/check-public-metadata.sh
• go test ./...

Closes #71

[github-actions]

Hi @Will-thom, thank you for contributing to PR Maven CLI.

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