Add a SKILL for adding and verifying copyright headers (#9779)

.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
+
+## 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.
+-->
+```
+
+### Shell Scripts (`.sh`)
+
+```bash
+# 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 based on its file extension.
+   - Replace `20??` with the current year (e.g., 2026).
+2. **Placement**:
+   - The copyright header should be at the very top of the file by default.
+   - **Exception**: If the file requires a shebang (e.g., `#!/bin/bash` in `.sh` files) or other necessary frontmatter, the copyright header must come **after** the frontmatter, separated by a blank line.
+3. **Exclusions**: Copyright headers are **not necessary** in directories that begin with a dot (`.`), such as:
+   - `.agents/`
+   - `.gemini/`
+   - `.github/`
+4. **Existing Files**: When editing an existing file, do NOT modify the year in the copyright header. Leave it as it is.
+5. **Subsequent Edits**: Subsequent edits should not modify the year.
+
+## Examples (for 2026)
+
+### Shell Script with Shebang
+```bash
+#!/bin/bash
+
+# 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.
+```
+
+### Dart File
+```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.
+```