feat: add startup config validation and document ConfigManager pipeline editor

Add ConfigurationValidationRunner with IConfigurationValidator interface for
validating required settings at startup. Includes SecureStore and LDAP validators.
Expand ConfigManager with pipeline editing UI, dialogs, and step editors.
Update documentation with config validation guidance.

NEW/src/JdeScoping.Core/Validation/IConfigurationValidator.cs

@@ -0,0 +1,24 @@
+namespace JdeScoping.Core.Validation;
+
+/// <summary>
+/// Interface for configuration validators that run on application startup.
+/// </summary>
+public interface IConfigurationValidator
+{
+    /// <summary>
+    /// Order in which this validator runs. Lower values run first.
+    /// Convention: 100=SecureStore, 200=LDAP, 300+=future validators.
+    /// </summary>
+    int Order { get; }
+
+    /// <summary>
+    /// Display name for logging purposes.
+    /// </summary>
+    string Name { get; }
+
+    /// <summary>
+    /// Validates the configuration and returns a result with any errors or warnings.
+    /// </summary>
+    /// <returns>Validation result containing errors and warnings.</returns>
+    ConfigurationValidationResult Validate();
+}