CI: rename CI README and add pull_request trigger to workflow

[fishingfly]

Summary

This PR improves the CI setup by renaming the CI README file to better reflect
its purpose and enabling the pull_request trigger in the GitHub Actions
workflow.

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Changes

• Rename CI-related README to a more conventional and descriptive name
• Add pull_request event to the CI workflow so tests run on PRs

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Motivation

• Use clearer and more standard naming for CI documentation
• Ensure CI checks are automatically executed for pull requests, improving
  code quality and review efficiency

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Impact

• CI only; no changes to runtime behavior
• No impact on public APIs

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Notes

The workflow logic itself remains unchanged. This PR only adjusts triggers and
documentation naming.