feat(server): provider switching API, WS/REST cancel, summarize endpoints

internal/agent/summarize.go

@@ -89,7 +89,7 @@ func (a *Agent) summarizeLocked(ctx context.Context, prov llm.Provider, sessionI
 	resp, err := prov.Chat(ctx, llm.Request{Messages: []llm.Message{
 		{Role: llm.RoleSystem, Content: summarizePrompt},
 		{Role: llm.RoleUser, Content: b.String()},
-	}}, nil)
+	}}, func(string) {})
 	if err != nil {
 		return nil, fmt.Errorf("summarize: %w", err)
 	}