`validateBranchName` rejects branch names containing `@`, even though
`git check-ref-format` permits `@` and GitHub itself accepts such
branches. PRs whose head or base branch contains an `@` fail validation
in-process before any git operation, so the action errors out
immediately.
Branch names with `@` show up in real workflows: ticket conventions
like "TICKET-123@add-feature" (#998), leading-prefix conventions like
"@hotfix/...", and agent tooling that appends "@<sessionid>" (#1305).
There is no workaround other than renaming the branch, which is often
not under the user's control.
Branch names are never passed through a shell (git calls use
execFileSync argv arrays), so `@` carries no injection risk. This is
the same reasoning used to add `#` in #1167, `+` in #1248, and `,` in
#1310. The bare name "@" (HEAD shorthand in git revision syntax) and
the "@{" reflog sequence are still rejected.
- Add `@` to the validateBranchName whitelist regex, including the
leading position (the leading-character rule blocks option injection
via `-`, which `@` cannot cause)
- Reject the bare name "@" with a dedicated check
- Update the surrounding comment, JSDoc, and error message to match
- Add test cases for @-containing names and bare "@"
Fixes#998
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
