Per-session workspace, tools only when workspace set

- Session has workspace field (absolute path, optional)
- ToolService stateless, receives workspace per call
- No workspace = no tools sent to API
- Removed hardcoded workspace from Program.cs
- Workspace field in session edit form

Endpoints/ChatEndpoint.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Net.Http;
 using System.Text.Json;
@@ -121,7 +122,11 @@ public static class ChatEndpoint
             new { type = "text", text = systemPromptText }
         };
 
+        var workspace = session.Workspace;
+        if (!string.IsNullOrEmpty(workspace))
+            Directory.CreateDirectory(workspace);
+
         var anthropic = new AnthropicService(httpFactory, token, toolService, db);
-        await anthropic.StreamChat(res, modelId, systemPrompt, messages, chainId);
+        await anthropic.StreamChat(res, modelId, systemPrompt, messages, chainId, workspace);
     }
 }