`validateBranchName` used a strict whitelist that excluded `#`,
causing the action to fail on PRs from branches like `put-back-arm64-#2`
with "Invalid branch name" — even though the branch already exists in
git and `#` is permitted by git-check-ref-format.
The validation was designed to prevent command injection. However, every
git call in the action uses `execFileSync`, which bypasses the shell
entirely and passes arguments directly to the kernel's execve. There is
no shell to interpret `#` as a metacharacter, so the strict whitelist was
over-blocking valid names with no security benefit.
Add `#` to the whitelist pattern, and update the JSDoc and error message
to reflect the allowed character set.
Fixes#1137.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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