tests: add issue-template fixture check

[karolswdev]

Goal

Add a small docs-gate fixture check that makes GitHub issue-template drift easier to catch.

Pointers

• Current docs gate: scripts/check-docs.mjs.
• Bug issue form: .github/ISSUE_TEMPLATE/bug_report.yml.
• Feature issue form: .github/ISSUE_TEMPLATE/feature_request.yml.

Acceptance

• npm run test:docs fails if either issue form is missing the asset-boundary checkbox.
• The check stays lightweight and does not require a YAML dependency unless there is a clear reason.
• Runs npm run test:docs.

No original DOS/Amiga data is needed for this task.