Tests: Add pre-commit link checking hook for changed files.

[brycelelbach]

Summary

• Add a test-links pre-commit hook that checks links in only the staged .md and .ipynb files, rather than the entire repo. Pre-commit passes the changed filenames directly to brev/test-links.bash.
• Refactor brev/test-links.bash to accept multiple file and directory arguments. Notebooks are converted individually; jupyter is only required when notebooks are among the targets.
• Rename the existing whole-repo link check to test-links-all at the manual stage (pre-commit run --hook-stage manual test-links-all).
• Document pre-commit hook setup and the hook table in CONTRIBUTING.md.