fix: resolve PR number extraction in forward-port workflow

[Crunchyman-ralph]

Summary

• Fix forward-port CI workflow failing with exit code 1
• Replace grep -oE '[0-9]+$' with basename to extract PR number from URL
• The $' sequence in the grep regex was being consumed by bash's ANSI-C quoting, breaking the command

Test plan

• Trigger the forward-port workflow and verify the PR creation step succeeds
• Verify PR_NUMBER is correctly extracted from the gh pr create output URL

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Note

Low Risk
Small change limited to CI workflow scripting; main risk is mis-parsing unexpected gh output formats, but basename is simpler and less error-prone than the previous regex.

Overview
Fixes the forward-port workflow’s PR-number extraction by replacing the grep -oE '[0-9]+$' parsing with basename "$PR_URL", avoiding bash/regex quoting issues that could cause the job to fail and preventing follow-up steps (like labeling) from breaking.

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Latest commit: 2c4725f

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