feat: use --id flag with branch name for stable preview URLs

[devin-ai-integration]

Summary

Passes --id "$HEAD_REF" to fern generate --docs --preview so that repeated pushes to the same PR branch reuse the same preview URL instead of generating a new one each time.

The branch name is passed via an environment variable (HEAD_REF) rather than inline ${{ github.head_ref }} interpolation to prevent shell injection from malicious branch names.

Also bumps the Fern CLI version from 0.37.15 to 4.35.0 so the --id flag is available (added in fern-api/fern#13660, released in 4.35.0).

Review & Testing Checklist for Human

• Version bump from 0.37.15 → 4.35.0: This is a large version jump. Verify that docs generation still works correctly with the new CLI version — there may be breaking changes or behavioral differences in the many intermediate releases.
• Branch names with /: github.head_ref can contain slashes (e.g. feature/my-update). Slashes are invalid in DNS labels. Verify the FDR server handles or sanitizes slashes in the preview ID.
• Test plan: Open a test PR against this repo, verify the preview URL uses the branch name. Push another commit to the same PR, verify the preview URL stays the same (not a new random UUID).

Notes

• The || true on the fern command means a CLI failure is silently swallowed — the PR comment will have a blank link. Worth keeping in mind for debugging.

Link to Devin session: https://app.devin.ai/sessions/d3670821970e4e1687c12f99240e043e

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[devin-ai-integration]

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