Replace jitterbit/get-changed-files with native git diff

[logan-stytch]

Summary

• Removes the jitterbit/get-changed-files@v1 action
• Replaces the two-step "get files + loop to check" pattern with a single step using git fetch --depth=1 + git diff --quiet against github.event.before/github.sha
• Matches the fix already shipped in stytch-node

Why

The jitterbit action was deprecated/unmaintained. The native git approach is simpler, has no external dependency, and correctly handles GitHub Actions' shallow clones via the explicit git fetch.

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