fix(ci): use docker login --password-stdin (bypass login-action@v3)

docker/login-action@v3 kept emitting "malformed HTTP Authorization
header" against a fresh, known-good dckr_pat_* token (verified by
direct curl against hub.docker.com/v2/users/login). Replacing with
`docker login --password-stdin` — Docker's documented credential
ingestion path — sidesteps whatever encoding the action injects.

Refs #794.

Co-Authored-By: claude-flow <ruv@ruv.net>