Safe outputs: push-to-branch for updating a pull request

[dsyme]

safe-outputs:
push-to-branch:
branch: feature-branch # Optional: the branch to push changes to (default: "triggering")
target: "*" # Optional: target for push operations
# "triggering" (default) - only push in triggering PR context
# "*" - allow pushes to any pull request (requires pull_request_number in agent output)
# explicit number - push for specific pull request number