Guard BillingModeField with isLocal check

Prevents the billing mode dropdown from rendering for non-local
adapters like openclaw_gateway where the server-side execute never
reads config.billingMode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/AgentConfigForm.tsx

@@ -774,7 +774,7 @@ export function AgentConfigForm(props: AgentConfigFormProps) {
                 <ClaudeLocalAdvancedFields {...adapterFieldProps} />
               )}
 
-              <BillingModeField {...adapterFieldProps} />
+              {isLocal && <BillingModeField {...adapterFieldProps} />}
 
               <Field label="Extra args (comma-separated)" hint={help.extraArgs}>
                 <DraftInput