Use @ as the separator between model and provider

Also put the provider after the model (e.g. `gpt-4o@openai`,
`openai/gpt-4o@openrouter`. The aim here was to reduce clashing and
confusion with existing model naming conventions.

pkg/lmcli/lmcli.go

@@ -66,7 +66,7 @@ func (c *Context) GetModels() (models []string) {
 	for _, p := range c.Config.Providers {
 		for _, m := range *p.Models {
 			modelCounts[m]++
-			models = append(models, *p.Name+"/"+m)
+			models = append(models, fmt.Sprintf("%s@%s", m, *p.Name))
 		}
 	}
 
@@ -80,12 +80,12 @@ func (c *Context) GetModels() (models []string) {
 }
 
 func (c *Context) GetModelProvider(model string) (string, api.ChatCompletionClient, error) {
-	parts := strings.Split(model, "/")
+	parts := strings.Split(model, "@")
 
 	var provider string
 	if len(parts) > 1 {
-		provider = parts[0]
-		model = parts[1]
+		model = parts[0]
+		provider = parts[1]
 	}
 
 	for _, p := range c.Config.Providers {