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);
     }
 }

Endpoints/SessionEndpoint.cs

@@ -50,8 +50,9 @@ public static class SessionEndpoint
         var description = body.TryGetProperty("description", out var d) ? d.GetString() : null;
         var image = body.TryGetProperty("image", out var img) ? img.GetString() : null;
         var agentId = body.TryGetProperty("agentId", out var a) ? a.GetString() : null;
+        var workspace = body.TryGetProperty("workspace", out var w) ? w.GetString() : null;
 
-        var session = await db.CreateSession(name, description, image, agentId);
+        var session = await db.CreateSession(name, description, image, agentId, workspace);
         await ctx.Response.WriteAsJsonAsync(session);
     }