Two fixes:
- Use BASE_BRANCH (with fallback) instead of DEFAULT_BRANCH for the
PR base field, preventing empty base causing 404
- Replace fixed sleep with branch readiness poll (up to 10 attempts)
and add retry loop for PR creation (up to 5 attempts), fixing
intermittent 404 when Gitea hasn't indexed the branch yet
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>