docs: add XML documentation and ConfigManager implementation plans

Add comprehensive XML documentation (param/returns tags) across 132 source
files to improve IntelliSense and API discoverability. Include ConfigManager
design documents and implementation plans for phases 1-9.

NEW/src/JdeScoping.Core/Models/Auth/LoginModel.cs

@@ -7,9 +7,15 @@ namespace JdeScoping.Core.Models.Auth;
 /// </summary>
 public class LoginModel
 {
+    /// <summary>
+    /// Gets or sets the user's login username.
+    /// </summary>
     [Required(ErrorMessage = "Username is required")]
     public string Username { get; set; } = string.Empty;
 
+    /// <summary>
+    /// Gets or sets the user's login password.
+    /// </summary>
     [Required(ErrorMessage = "Password is required")]
     public string Password { get; set; } = string.Empty;
 }