Replace the metaschema-xslt toolchain and Maven/Saxon infrastructure
with oscal-cli-enhanced for all schema generation and validation tasks.

Build system changes:
- Add oscal-cli auto-install that checks PATH first, downloads from
  Maven Central to build/bin/ if not found
- Replace XSLT-based schema generation with oscal-cli metaschema
  generate-schema for both JSON Schema and XSD output
- Replace schematron composition validation with oscal-cli metaschema
  validate
- Remove XSLT converter generation (oscal-cli provides direct content
  conversion)
- Remove resolved metaschema generation (no longer needed)
- Update archive generation to include schemas and source metaschemas
- Make xmllint optional for environments without it
- Move link checking to end of build pipeline

Dependency removal:
- Remove metaschema-xslt submodule and nested schxslt submodule
- Remove Maven pom.xml, resolve-entities.xsl, and xslt-runner.sh
- Remove Java setup from CI/CD status workflow

Documentation:
- Update build README with oscal-cli prerequisites and usage
- Replace XSLT converter documentation with oscal-cli content
  conversion examples
- Update release README to reflect new archive structure