feat: add GitHub Actions workflow to run lint on PRs

[q-soriarty]

Problem

scripts/lint.sh exists but only runs manually. A contributor can merge breaking changes (version mismatch, broken heading hierarchy) without the lint catching it.

Expected behavior

Add .github/workflows/lint.yml that:

• Triggers on pull requests targeting main and develop
• Runs ./scripts/lint.sh
• Fails the PR check if any lint errors are found
• Lightweight: no dependencies, just bash

This closes the gap between having a linter and enforcing it.

[github-actions]

Closed by #83 (merged into develop).