fn provider_install_dir(provider_id: &str) -> Option<PathBuf>Expand description
Canonical install directory for a provider —
<agentmux_home>/instances/v<version>/cli/<provider>/. This is the
same path the frontend uses to launch the agent
(agent-model.ts::resolveCliDir), so the bin we drop in
node_modules/.bin/ is what the launch path will execute.
Honors portable / installed mode + the AGENTMUX_HOME_OVERRIDE
test override via DataPaths::from_env().