feat(skills): add cleanup-artifacts skill and session hygiene protocol

bin/validate-skills

@@ -51,7 +51,7 @@ def metadata(text):
 
 def local_links(path):
     fenced = False
-    for number, line in enumerate(path.read_text().splitlines(), 1):
+    for number, line in enumerate(path.read_text(encoding='utf-8').splitlines(), 1):
         if line.lstrip().startswith(('```', '~~~')):
             fenced = not fenced
             continue