fix: give issue reporters field-specific guidance

[marvinctl]

Type of change(s)

• Infrastructure / tooling (CI, build, deps, tests)

What changed and why

• Identify missing or incomplete required issue-form fields and give reporters field-specific guidance.
• Validate classifier field output and keep the existing generic response as a fallback.
• Mark policy comments with their result so stale labels and comments can be updated safely, including existing comments without result markers.

Validation

• JavaScript syntax and whitespace checks.
• Manual checks covered required-field extraction, field validation, rendered guidance, fallback behavior, duplicate suppression, and legacy comment handling.

Checklist

• I read the contribution guidelines.
• I have self-reviewed my pull request.
• I understand every change in this pull request.