fix(ci): extract inline Python to script for validate-config workflow

The inline Python in playbook-schema job had YAML parsing issues.
Extracted to scripts/validate_playbook_schema.py.

Refs: #461

scripts/validate_playbook_schema.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+"""Validate playbook YAML files have required keys."""
+import yaml
+import sys
+import glob
+
+required_keys = {'name', 'description'}
+
+for f in glob.glob('playbooks/*.yaml'):
+    with open(f) as fh:
+        try:
+            data = yaml.safe_load(fh)
+            if not isinstance(data, dict):
+                print(f'ERROR: {f} is not a YAML mapping')
+                sys.exit(1)
+            missing = required_keys - set(data.keys())
+            if missing:
+                print(f'WARNING: {f} missing keys: {missing}')
+            print(f'OK: {f}')
+        except yaml.YAMLError as e:
+            print(f'ERROR: {f}: {e}')
+            sys.exit(1)