Branch names starting with an underscore (e.g. _release/v1.2.3) are valid
per git check-ref-format but were rejected by validateBranchName's
first-character whitelist. Since setupBranch validates a PR's baseRefName
after checkout, the action failed on every open PR targeting such a
branch. A leading underscore carries no option-injection risk (only a
leading dash does, which is still rejected separately).
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>