Selecting a model on the copilot-acp provider had no effect: the model id
never left Hermes. _create_chat_completion() dropped the model argument
before _run_prompt(), so the selection survived only as prompt text
('Hermes requested model hint: ...') and Copilot answered with its own
session default — a user picking gpt-5.6-terra visibly got Claude Sonnet 5.
Live-probing 'copilot --acp --stdio' shows the CLI validates but IGNORES
its --model spawn flag in ACP mode, while session/new advertises
models.availableModels and the ACP-native session/set_model call actually
switches the session. Wire that in: forward the model into _run_prompt,
and after session/new send session/set_model when the id is advertised
(or the server reports no list). Unknown ids degrade to the session
default with a warning instead of failing the turn; the provider-level
virtual slug 'copilot-acp' is never forwarded.
Verified live against the real CLI: requesting gpt-5.6-terra answers as
GPT-5.6 Terra and claude-sonnet-5 answers as Claude Sonnet 5.
unsupportedpastels
426dab7de0