Expand teaching markdown CI gate to cover shared teaching docs

[VatsalSy]

Why

Markdown/format regressions in teaching content were repeatedly identified during review.

Evidence

• PR maintenance-updates #59 review: Prettier/markdownlint failures in teaching markdown and teaching asset docs.
• PR Harden content and script maintenance safeguards #62 added teaching-content checks, but scope should include all teaching-adjacent markdown touched in practice.

Scope

• Confirm and expand CI lint scope to include:
  • _teaching/**/*.md
  • assets/images/teaching/README.md
  • Any other teaching-adjacent markdown with recurring style issues.
• Ensure both prettier --check and markdownlint run on this scope.
• Document the exact local command in contributor docs for fast pre-PR validation.

Acceptance Criteria

• CI fails on formatting/lint issues for the agreed teaching markdown scope.
• Local one-liner command for same checks is documented.
• No false-positive exclusions are added to bypass real markdown errors.