feat: add push-to-pull-request-branch to issue-implementer for CI fix iteration #34
Description
The issue-implementer workflow currently creates PRs but can't iterate on them when CI fails or reviews come in. Adding the push-to-pull-request-branch safe output would let it push fixes to its own PRs.
This would enable a self-healing loop: implementer opens PR → CI fails → implementer reads failure → pushes fix to same PR branch.