ci: run latexmk on pull requests to master, closes #16

[bibonix]

Closes #16. The latexmk workflow was the only check in .github/workflows/ that did not run on pull_request events, so pull requests against master could pass every other check and still break the latexmk -pdf build that produces brand-book.pdf.

The change adds pull_request: branches: [master] next to the existing push: branches: [master] trigger in .github/workflows/latexmk.yml. The JamesIves/github-pages-deploy-action step keeps its if: github.ref == 'refs/heads/master' guard, so the gh-pages deploy is still gated to master pushes; pull requests now exercise the build path through the cp brand-book.pdf gh-pages step but do not publish.

Validated locally by running yamllint .github/workflows/latexmk.yml against the modified file (no output) and by re-reading the diff to confirm the deploy guard is unchanged. CI on the pull request itself will exercise the full workflow set, including the now-triggered latexmk job.

[GHX5T-SOL]

Approved current head 750e686640c36c1a894da533bc03f438cd22025c.

The diff is limited to .github/workflows/latexmk.yml and adds the missing pull_request trigger for master while preserving the existing push trigger and the deploy step guard if: github.ref == 'refs/heads/master', so the PDF build now runs on PRs without deploying from PR events.

Validation/readback:

• GitHub actionlint, bibcop, copyrights, markdown-lint, pdd, reuse, typos, xcop, and yamllint checks are successful on this head.
• The PR latexmk run 26390650683 fails in yegor256/latexmk-action@0.19.0 because fontspec cannot find molot-regular.
• The latest master latexmk run 26386955857 fails with the same molot-regular font error, so this is an existing baseline/toolchain failure exposed by the new PR trigger, not caused by the trigger-only patch.
• Local YAML parse with Ruby YAML.load_file -> OK.
• git diff --check origin/master...HEAD -> clean.
• Diff Gitleaks -> no leaks.

Local actionlint and yamllint binaries are not installed here, so I relied on the successful GitHub checks for those two validators.

[0crat]

@GHX5T-SOL Great work on the review! 🎯 You've earned +6 points (base +18, with deductions for missing comments and limited code coverage of 3 hits). Your total score is now +206 - keep up the momentum and check your Zerocracy account for updates!

[VasilevNStas]

@bibonix the latexmk build fails because the branch was created before the wget -O molot-regular.ttf step was added to master (in PR #50). Since this PR modifies .github/workflows/latexmk.yml, the merge commit uses the branch version of the file — the one without the font download step.

Fix: rebase this branch on the latest master to pick up the font download step:

git fetch zerocracy master
git rebase zerocracy/master
git push --force-with-lease

Then CI should pass.