Add copyright headers skills

Author: elliette

.agents/skills/adding-copyright-headers/SKILL.md

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+---
+name: adding-copyright-headers
+description: Adds copyright headers to files based on file type. Use when creating new files or when asked to verify copyright headers.
+---
+
+# Adding Copyright Headers
+
+This skill provides instructions for adding copyright headers to source files in this repository based on their file type.
+
+## Formats
+
+### Dart Files (`.dart`)
+
+```dart
+// Copyright 20?? The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+```
+
+### YAML Files (`.yaml`)
+
+```yaml
+# Copyright 20?? The Flutter Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+```
+
+### HTML Files (`.html`) and Markdown Files (`.md`)
+
+```html
+<!--
+Copyright 20?? The Flutter Authors
+Use of this source code is governed by a BSD-style license that can be
+found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+-->
+```
+
+## Instructions
+
+1. **New Files**: When creating a new file, you MUST add the appropriate copyright header at the very top of the file based on its file extension.
+   - Replace `20??` with the current year (e.g., 2026).
+2. **Existing Files**: When editing an existing file, do NOT modify the year in the copyright header. Leave it as it is.
+3. **Subsequent Edits**: Subsequent edits should not modify the year.
+
+## Examples (for 2026)
+
+### Dart
+```dart
+// Copyright 2026 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+```
+
+### YAML
+```yaml
+# Copyright 2026 The Flutter Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+```
+
+### HTML / Markdown
+```html
+<!--
+Copyright 2026 The Flutter Authors
+Use of this source code is governed by a BSD-style license that can be
+found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+-->
+```