ci: pin Protocol verification actions

tools/check_ci_actions.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+"""Require immutable commits for third-party Gitea workflow actions."""
+
+from __future__ import annotations
+
+import pathlib
+import re
+
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+USE = re.compile(r"^\s*-\s+uses:\s+([^@\s]+)@([^\s#]+)", re.MULTILINE)
+
+
+for workflow in sorted((ROOT / ".gitea/workflows").glob("*.y*ml")):
+    for action, revision in USE.findall(workflow.read_text(encoding="utf-8")):
+        if not action.startswith("./") and not re.fullmatch(r"[0-9a-f]{40}", revision):
+            raise SystemExit(f"{workflow.relative_to(ROOT)}: mutable action {action}@{revision}")
+
+print("Protocol CI action references are immutable")