Reverts the following commits:
- f669191 fix: use unique local branch names for PR checkout to avoid conflicts (#931)
- 21e3fe0 Fix PR checkout to support fork PRs (#851)
Simplifies PR branch checkout back to using headRefName directly instead
of the pr-{number} local branch naming scheme introduced in #931 and the
GitHub pull ref fetch approach introduced in #851.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>