[agents] Add rule to enforce license header on new Dart files

Enforces the inclusion of the standard copyright and BSD-style
license header with the current year at the top of newly created Dart files.

TAG=agy
CONV=03fa0bca-5bc4-4526-8592-707d62f64410

Change-Id: I0b4a085e3f2d69a3459dee84336f0be36499e4cb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/509440
Reviewed-by: Kevin Moore <kevmoo@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

.agents/rules/add_license_header.md

@@ -0,0 +1,21 @@
+---
+trigger: glob
+description: Ensure standard Dart license header on newly created Dart files.
+globs: *.dart
+---
+
+# License Header Rule
+
+Whenever you create a new Dart (`.dart`) file in this repository, you MUST include the standard Dart project license header at the very top (line 1).
+
+Replace `<CURRENT_YEAR>` with the actual current year (e.g., 2026).
+
+```dart
+// Copyright (c) <CURRENT_YEAR>, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+```
+
+## Requirements
+- Insert at line 1.
+- Ensure a blank line follows the license header before any other code or comments.