Isolate reproduction publishing credentials from agent execution (#21106)
The regression-test agent currently shares its job with `id-token:
write`, allowing untrusted code to mint repository-wide `uv-dev`
credentials before the intended token exchange. Move publication into a
separate trusted job that imports an immutable Git bundle, verifies its
single parent and integration-test-only changes, and exchanges OIDC
credentials only after the proposed commit has been validated.
Co-authored-by: Zanie Blue <contact@zanie.dev>