fix: persist -m model when switching agents

Add initial model from command line to fallback chain so it persists
when switching agents with tab, matching behavior of config model.

Resolves #3863

packages/opencode/src/cli/cmd/tui/context/local.tsx

@@ -150,6 +150,16 @@ export const { use: useLocal, provider: LocalProvider } = createSimpleContext({
         })
 
       const fallbackModel = createMemo(() => {
+        if (props.initialModel) {
+          const { providerID, modelID } = Provider.parseModel(props.initialModel)
+          if (isModelValid({ providerID, modelID })) {
+            return {
+              providerID,
+              modelID,
+            }
+          }
+        }
+
         if (sync.data.config.model) {
           const { providerID, modelID } = Provider.parseModel(sync.data.config.model)
           if (isModelValid({ providerID, modelID })) {