/moa no longer does a sticky model switch. It now always runs a single
prompt through the default MoA preset and restores the prior model
afterward; the whole argument is the prompt (no preset-name matching).
To switch to a MoA preset for the session, select it from the model
picker, where presets already surface under a virtual Mixture of Agents
provider on every model-selection surface.
Also fixes#53444: the TUI one-shot only set session[model_override],
which the already-built cached agent ignored, so MoA silently never ran
and the turn used the original model. The TUI now does a real in-place
agent.switch_model() via _apply_model_switch() when a live agent exists
(with a proper restore after the turn), and falls back to a model_override
for lazy/unbuilt sessions.
Removes the redundant sticky-switch branch from the CLI, gateway, and TUI
/moa handlers; updates the command description, usage string, and docs.
50f6855217