Function validate_content_dir 

pub fn validate_content_dir(
    content_dir: &Path,
    schemas: &[ContentSchema],
) -> Result<Vec<ValidationError>>

Validates every .md file in content_dir against the loaded schemas.

Files opt in by including a schema = "<name>" field in their frontmatter. Files without the field are silently skipped.

Returns all validation errors found across all files.