Exposes self-hosted n8n chat agents behind /v1/chat/completions and
/v1/models. Model -> n8n webhook routing via a GenServer registry, so
multiple agents map to multiple models. Plug + Bandit, req for the
n8n webhook call, Bearer auth (ADAPTER_API_KEY). Ships a flake.nix
(beamPackages.mixRelease) so it can be consumed as a NixOS flake input.
