test(docs): guard the signed marketplace contribution policy

tests/test_contributing_plugin_policy.py

@@ -0,0 +1,47 @@
+"""Regression checks for the plugin contribution policy (#427).
+
+The contributing guide must describe the signed, allowlisted catalog model and
+must never reintroduce the obsolete blanket "local only" rule. These checks make
+the policy testable and keep a future edit from silently reverting it.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+def _contributing() -> str:
+    text = (REPO_ROOT / "CONTRIBUTING.md").read_text(encoding="utf-8").lower()
+    return " ".join(text.split())
+
+
+def test_contributing_has_no_blanket_local_only_rule():
+    text = _contributing()
+
+    for stale in ("local only", "local-only", "local/trusted"):
+        assert stale not in text, f"CONTRIBUTING.md still contains the stale rule {stale!r}"
+
+
+def test_contributing_describes_signed_allowlisted_installs():
+    text = _contributing()
+
+    for required in (
+        "signed and allowlisted",
+        "plugin_catalog_file",
+        "plugin_marketplace_url",
+        "plugin_trust_policy",
+        "ed25519",
+        "integrity",
+        "never accept a client-supplied artifact url",
+    ):
+        assert required in text, f"CONTRIBUTING.md is missing the plugin policy detail {required!r}"
+
+
+def test_contributing_keeps_no_implicit_grants_and_no_code_execution():
+    text = _contributing()
+
+    assert "never grant capabilities implicitly" in text
+    assert "never make the management api/cli load" in text
+    assert "or execute plugin code" in text