Lint rule: required front matter fields

[roxspring]

Implement a lint rule that reports when a .md file is missing required front matter fields.

Scope

• Configurable list of required field names (e.g. ["title", "weight"])
• If a file has no front matter at all and required fields are configured, report a violation
• If a file has front matter but is missing one or more required fields, report each missing field as a separate violation
• Report file path for each violation

Example violation

guide/editor.md: missing required front matter field: 'weight'

Acceptance criteria

• File with all required fields: no violation
• File missing one field: one violation reported
• File with no front matter: violation reported (when required fields are configured)
• Empty required fields list: no violations