ci: install pyyaml in validate-manifest workflow

scripts/skills.py grew `import yaml` in db9b2e5 to parse SKILL.md
frontmatter, but the workflow doesn't pip install it, so validate has
been failing with ModuleNotFoundError on every push since.

Co-authored-by: Isaac

Author: jamesbroadhead

.github/workflows/validate-manifest.yml

@@ -25,5 +25,8 @@ jobs:
         with:
           python-version: '3.11'
 
+      - name: Install Python deps
+        run: pip install pyyaml
+
       - name: Validate manifest is up to date
         run: python3 scripts/skills.py validate