An ACP agent's model and effort pickers now list what its session offers
(its `model` and `thought_level` config options) instead of the claude
model list and EFFORT_LEVELS. A pick goes through the same Bus::set_model /
Bus::set_effort -> Config.model / Config.effort path as on claude; before
each prompt the ACP runtime sets it with `session/set_config_option`, model
first, and only when the session offers that value. Options are re-read
from the set response and from `config_option_update`, so the effort picker
disappears when the chosen model offers no effort levels, and is hidden
while a newly picked model waits for the next turn.
The session's options reach the web UI through a `Choices` handle from a
new `Runtime::choices`, registered on the bus the way `canceller` is.
/api/model and /api/effort accept only offered values on ACP. The claude
path is unchanged.
Refs #4391
f189724a4c