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

[devin-ai-integration]

Summary

Updates the preview-docs.yml workflow to pass --id with the PR branch name to fern generate --docs --preview. This makes the preview URL stable across repeated pushes to the same PR branch — instead of generating a new random UUID each time, the same branch name produces the same preview URL.

Also bumps the Fern CLI version from 4.15.5 → 4.35.0 so the version redirection uses a release that includes --id support.

Depends on: fern-api/fern#13660 (adds --id flag to CLI, merged) and fern-api/fern-platform#8742 (FDR server support, merged).

Review & Testing Checklist for Human

• Version bump 4.15.5 → 4.35.0: This is a significant version jump. Verify that docs generation still works correctly with the new CLI version — there may be breaking changes or behavioral differences unrelated to --id.
• Branch names with / or special characters: github.head_ref can contain slashes (e.g. feature/my-docs-update). These get embedded into a DNS subdomain like {org}-preview-{id}.docs.buildwithfern.com. Slashes are invalid in DNS labels. Verify the FDR server sanitizes or rejects these gracefully.
• Test plan: Open a test PR, verify the preview URL uses the branch name as the ID. Push another commit to the same PR, verify the preview URL stays the same and the old preview is overwritten.

Notes

• Shell injection is mitigated: github.head_ref is passed via an env variable (HEAD_REF) rather than inline ${{ }} interpolation in the run block, so it's treated as data by bash.
• Tested the --id flag end-to-end against the dev stack using fern-dev:build — confirmed it generates stable preview URLs (e.g. smoke-test-preview-my-test-preview.docs.dev.buildwithfern.com).
• The || true on the fern command means a CLI failure would be silently swallowed — the URL grep would just produce empty output and the PR comment would have a blank link.

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

[devin-ai-integration]

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[github-actions]

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