Adding a staging skill (#13166)

As it says

Author: sfshaza2

.agents/skills/validate-pr/SKILL.md

@@ -0,0 +1,49 @@
+---
+name: validate-pr
+description: >-
+  Stages the Flutter site locally. This includes checking for broken links,
+  verifying excerpts, updating formatting, and serving the dev site.
+---
+
+# Stage the Flutter site
+
+Before committing changes or reviewing a PR locally,
+it is important to stage the site and ensure everything works correctly. 
+Follow these steps to stage the site:
+
+## 1. Sync code excerpts
+
+Ensure that any changed code excerpts are properly run and synced to the
+Markdown files:
+
+```bash
+dart run dash_site refresh-excerpts
+```
+
+## 2. Update formatting
+
+Update the formatting of the site examples and tools to ensure
+they follow the official Dart format:
+
+```bash
+dart run dash_site format-dart
+```
+
+## 3. Check for broken links
+
+Run the following command to check for any unlinked or broken
+Markdown link references in the site output:
+
+```bash
+dart run dash_site check-link-references
+```
+
+If any broken links are found, try to patch them or alert the user.
+
+## 4. Stage the site locally
+
+Finally, serve a local development server of the site so the user can preview the changes:
+
+```bash
+dart run dash_site serve
+```