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

Models/SessionRecord.cs

@@ -8,6 +8,7 @@ public class SessionRecord
     public string Description { get; set; }
     public string Image { get; set; }
     public string AgentId { get; set; }
+    public string Workspace { get; set; }
     public bool IsDeleted { get; set; }
     public string CreatedAt { get; set; }
     public string UpdatedAt { get; set; }