ci: check newly added links

[Himanshuagrawal4]

Summary

• save the main-branch linkcheck output as a cache baseline
• restore that baseline for pull requests and merge queues
• ignore only exact URLs already present in the baseline so newly introduced links are still checked
• tolerate incomplete JSONL cache records

Closes #2037

Validation

• actionlint .github/workflows/test.yml
• ruff check source/conf.py
• ruff format --check source/conf.py
• synthetic baseline parsing test, including malformed JSONL input

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📚 Documentation preview 📚: https://python-packaging-user-guide--2109.org.readthedocs.build/en/2109/

[webknjaz]

Why didn't you just reopen #2067?

[webknjaz]

Based on the account activity in my other projects, other maintainers' reactions in other projects and overall unauthentic interactions, I believe we don't need to invest effort in reviewing or encouraging similar-quality submissions from this account.

I'll leave these external links here for now but we should probably adopt s more formal policy in the future:

• https://palletsprojects.com/contributing/llm-ai
• https://dontquotetheai.com
• [policy] Good-faith agentic contributions and LLM use, and avoiding death by a thousand AI slops jazzband/pip-tools#2278 / Introduce an initial LLM Usage Policy jazzband/pip-tools#2318 / https://pip-tools.rtfd.io/en/stable/contributing/#llm-generated-contributions