♻️ refactor(skills): update playbook.py and tests for thirdparty/ layout

- install_skills_action: collect skills from both codex/skills/ (own) and
  codex/skills/thirdparty/ (thirdparty); mode=all installs both, mode=list
  searches both; log [thirdparty] tag for third-party installs
- Update test_thirdparty_skills_pipeline: fix source_list path assertion
- Update SKILLS.md: document thirdparty/ subdirectory structure

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/playbook.py

@@ -1141,6 +1141,7 @@ def install_skills_action(config: dict, context: dict) -> int:
         agents_home = (context["project_root"] / agents_home).resolve()
 
     skills_src_root = PLAYBOOK_ROOT / "codex/skills"
+    skills_thirdparty_root = skills_src_root / "thirdparty"
     if not skills_src_root.is_dir():
         print(f"ERROR: skills source not found: {skills_src_root}", file=sys.stderr)
         return 2
@@ -1149,28 +1150,40 @@ def install_skills_action(config: dict, context: dict) -> int:
     ensure_dir(skills_dst_root)
 
     if mode == "all":
-        skills = [
-            path.name
+        own_skills = [
+            (path.name, skills_src_root, "own")
             for path in skills_src_root.iterdir()
-            if path.is_dir() and not path.name.startswith(".")
+            if path.is_dir() and not path.name.startswith(".") and path.name != "thirdparty"
         ]
+        third_skills = [
+            (path.name, skills_thirdparty_root, "thirdparty")
+            for path in skills_thirdparty_root.iterdir()
+            if path.is_dir() and not path.name.startswith(".")
+        ] if skills_thirdparty_root.is_dir() else []
+        skill_entries = own_skills + third_skills
     elif mode == "list":
         try:
-            skills = normalize_names(config.get("skills"), "skills")
+            names = normalize_names(config.get("skills"), "skills")
         except ValueError as exc:
             print(f"ERROR: {exc}", file=sys.stderr)
             return 2
+        skill_entries = []
+        for name in names:
+            if (skills_src_root / name).is_dir():
+                skill_entries.append((name, skills_src_root, "own"))
+            elif skills_thirdparty_root.is_dir() and (skills_thirdparty_root / name).is_dir():
+                skill_entries.append((name, skills_thirdparty_root, "thirdparty"))
+            else:
+                print(f"ERROR: skill not found: {name}", file=sys.stderr)
+                return 2
     else:
         print("ERROR: mode must be list or all", file=sys.stderr)
         return 2
 
     timestamp = datetime.now().strftime("%Y%m%d%H%M%S")
     no_backup = bool(config.get("no_backup", False))
-    for name in skills:
-        src = skills_src_root / name
-        if not src.is_dir():
-            print(f"ERROR: skill not found: {name}", file=sys.stderr)
-            return 2
+    for name, src_root, origin in skill_entries:
+        src = src_root / name
         dst = skills_dst_root / name
         if dst.exists():
             if no_backup:
@@ -1181,7 +1194,8 @@ def install_skills_action(config: dict, context: dict) -> int:
                 log(f"Backed up existing skill: {name} -> {backup.name}")
         copytree(src, dst)
         rewrite_skill_docs_links(dst, resolve_docs_prefix(context))
-        log(f"Installed: {name}")
+        tag = " [thirdparty]" if origin == "thirdparty" else ""
+        log(f"Installed: {name}{tag}")
 
     return 0