fix: exclude star-history API from lychee link checker

[Copilot]

https://api.star-history.com/svg?repos=commitizen-tools/commitizen intermittently returns HTTP 500, causing the broken links CI check to fail. The service is externally owned and not in our control.

Description

Added https://api.star-history.com/ to the exclude list in lychee.toml to suppress false-positive broken link failures from this unreliable external service.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (GitHub Copilot)

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

The broken links CI check passes without flagging the star-history API image URL.

Steps to Test This Pull Request

1. Trigger the broken links workflow — it should no longer report an error for https://api.star-history.com/svg?repos=commitizen-tools/commitizen.

Additional Context

The star-history widget in docs/README.md (line 308) embeds an SVG from api.star-history.com. That service is currently returning 500, which is out of our control. Excluding the domain prefix covers this and any future star-history embed variants.

[github-actions]

🔍 Commitizen bump preview

No commits in this PR are eligible for a version bump.

[Lee-W]

we could rebase and rewrite the commit history as ci: otherwise, it'll bump a patch version

[bearomorphism]

Sorry... I forgot to check the prefix of the title