Derive provider from model ID

The provider is always the prefix before "/" in the model ID,
so storing it as a separate field is redundant. Replace the
field with a Provider() method.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/llm/registry.go

@@ -24,7 +24,6 @@ type (
 	ModelDefinition struct {
 		ID              string
 		Name            string
-		Provider        string
 		ContextLength   int
 		MaxOutputTokens int
 		Supports        SupportedParameters
@@ -89,6 +88,13 @@ func (r *Registry) Lookup(modelID string) (ModelDefinition, bool) {
 	return ModelDefinition{}, false
 }
 
+// Provider returns the provider prefix from the model ID (e.g. "anthropic"
+// from "anthropic/claude-opus-4.6").
+func (m *ModelDefinition) Provider() string {
+	provider, _, _ := strings.Cut(m.ID, "/")
+	return provider
+}
+
 func (r *Registry) index(m *ModelDefinition) {
 	r.byID[m.ID] = m