fix: allow check-warp-deploy to run on fork PRs

[paulbalaji]

Summary

• Skip GitHub App token generation for fork PRs (they don't have access to secrets)
• Write summary to GITHUB_STEP_SUMMARY so it's visible on workflow run page for all PRs
• PR comments still posted for non-fork PRs

Fixes: https://github.com/hyperlane-xyz/hyperlane-registry/actions/runs/21583871563/job/62225555469

Test plan

• Verify workflow passes on this PR (non-fork)
• Re-run the failed fork PR to confirm it works
• Check workflow summary appears on run page

🤖 Generated with Claude Code

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch pb/3p-checkwarpdeploy

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