fix: create beta release branches with valid git syntax

[heimanba]

Summary

• replace invalid git checkout --branch invocations with git checkout -b
• unblock the Prepare Beta Release workflow on GitHub-hosted runners

Behavior / risk

The existing workflow always fails before creating a beta branch because git checkout does not support --branch. This changes only branch creation syntax; branch names, tracking configuration, version calculation, and publishing gates remain unchanged.

Validation

• reproduced the failure in Prepare Beta Release runs 29733176912 and 29733250640
• git diff --check
• push verification profile: typecheck, architecture checks, tests, and changed-file lint