chore(pre-commit): validate marketplace manifest on pre-push

## Why
The root marketplace.json is the index that ties all plugins together.
claude plugin validate --strict covers schema-level checks that
check-manifests.sh does not, so it warrants its own hook alongside
validate-plugins.

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

.pre-commit-config.yaml

@@ -61,6 +61,15 @@ repos:
         pass_filenames: false
         always_run: true
 
+      - id: validate-marketplace
+        name: Validate marketplace manifest
+        description: Run claude plugin validate --strict on the root marketplace manifest
+        entry: claude plugin validate --strict .claude-plugin/marketplace.json
+        language: system
+        stages: [pre-push]
+        pass_filenames: false
+        always_run: true
+
       - id: skill-frontmatter
         stages: ['pre-commit']
         name: SKILL.md frontmatter validation