Add CI check against conventional commit messages

[SMillerDev]

Since we don't use conventional commit messages, it would be good to warn people about using them

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

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• AI was used to generate or assist with generating this PR.

Claude generated the whole workflow for me.

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[Copilot]

Pull request overview

This PR introduces a GitHub Actions check intended to detect and prevent (or at least flag) Conventional Commits-style prefixes (e.g., feat:, fix:) in pull request commit titles, aligning commit history with Homebrew’s preferred commit message conventions.

Changes:

• Adds a new GitHub Actions workflow that scans PR commit subjects between the PR base and head SHAs.
• Fails the workflow if any commit title matches a Conventional Commits-like prefix pattern.

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[MikeMcQuaid]

Looks good when comments addressed!

[Bo98]

This will probably flag Dependabot though we can reintroduce the commit message config we had before: https://github.com/Homebrew/ci-orchestrator-private/blob/300aac6dd98c70b0d8f41dfcde2f99b539825a39/.github/dependabot.yml#L27-L28