feat(tools): add internal relative link checker and fix dead links

[mjmirza]

Implements tools/check-links.py and unit tests in tools/check-links-test.py to automatically validate relative internal Markdown links across all documentation and pattern files in CI. Resolves existing broken internal links in README.md and patterns/04-principles-and-laws/predictable.md, and integrates the link checker into the Makefile and GitHub Actions workflow.

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PR created automatically by Jules for task 16841915612650116180 started by @mjmirza

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