Add ConditionalOn attribute

src/StaticPHP/Attribute/Package/ConditionalOn.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace StaticPHP\Attribute\Package;
+
+/**
+ * Makes a #[BeforeStage] or #[AfterStage] hook conditional on DI container bindings.
+ *
+ * The hook is only invoked when ALL specified classes are currently bound in the
+ * DI container. Multiple #[ConditionalOn] attributes on the same method use AND
+ * semantics — every condition must hold for the hook to run.
+ *
+ * Example:
+ *
+ *   #[ConditionalOn(PgoContext::class)]
+ *   #[BeforeStage('php', 'build')]
+ *   public function injectPgoFlags(PgoContext $ctx): void { ... }
+ */
+#[\Attribute(\Attribute::TARGET_METHOD | \Attribute::TARGET_CLASS | \Attribute::IS_REPEATABLE)]
+readonly class ConditionalOn
+{
+    /**
+     * @param class-string $class the class that must be present in the DI container for this hook to run
+     */
+    public function __construct(public string $class) {}
+}