KAN-73: document and validate MCP ticket description format

app/Rules/VisualTicketDescription.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Rules;
+
+use App\Actions\Tickets\Support\TicketDescriptionFormat;
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+
+class VisualTicketDescription implements ValidationRule
+{
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        if (! is_string($value)) {
+            return;
+        }
+        $unsupported = TicketDescriptionFormat::unsupportedContent($value);
+        if ($unsupported !== []) {
+            $fail('The description contains unsupported '.implode(', ', $unsupported).'. Remove these contents or show literal examples inside code. Use ordinary Markdown and safe links; put acceptance criteria only in acceptance_criteria. No ticket fields were changed.');
+        }
+    }
+}