feat: add command usage examples

Add an Examples string array property to CommandAttribute so command authors can document example invocations. Each example contains only the arguments that follow the command name; the executable and command name are prepended automatically when the example is rendered.

Examples are rendered as extra lines in the USAGE section of the help text. The source generator resolves the values from the attribute and emits them into the command descriptor consumed at runtime.

CliFx/Help/HelpWriter.cs

@@ -73,6 +73,19 @@ internal class HelpWriter(HelpContext context, ConsoleWriter consoleWriter)
             WriteLine();
         }
 
+        // Examples
+        foreach (var example in context.Command.Examples)
+        {
+            WriteHorizontalMargin();
+
+            WriteCommandInvocation();
+            Write(' ');
+
+            Write(example);
+
+            WriteLine();
+        }
+
         // Child commands
         if (context.Root.GetChildCommands(context.Command.Name).Any())
         {