Auto-create PRs for automated Seer handoffs

[JoshFerge]

Summary

• force automated Seer completion handoffs to request PR creation
• leave the lower-level handoff helper's preference/default behavior intact for non-automated callers
• add a regression test proving automated handoff ignores a false stored auto-create preference

Testing

• git diff --check
• push hooks: mypy, knip, whitespace, large-file checks

Local pytest/prek were not run because this worktree does not have .venv installed; CI will cover the test run.