chore(pre-commit): validate plugins on pre-push

## Why
`claude plugin validate --strict` catches structural issues in plugin
manifests that `check-manifests.sh` does not cover (e.g. schema
violations, unrecognised fields). Running it at pre-push ensures all
plugins in `plugins/*/` stay valid before reaching the remote.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.pre-commit-config.yaml

@@ -52,6 +52,15 @@ repos:
         pass_filenames: false
         always_run: true
 
+      - id: validate-plugins
+        name: Validate plugins
+        description: Run claude plugin validate --strict on every plugin directory
+        entry: bash -c 'for d in plugins/*/; do claude plugin validate --strict "$d" || exit 1; done'
+        language: system
+        stages: [pre-push]
+        pass_filenames: false
+        always_run: true
+
       - id: skill-frontmatter
         stages: ['pre-commit']
         name: SKILL.md frontmatter validation