docs: codify merge proof standard

tests/test_proof_policy_docs.py

@@ -0,0 +1,17 @@
+from pathlib import Path
+
+
+def test_contributing_sets_hard_proof_rule() -> None:
+    doc = Path("CONTRIBUTING.md").read_text()
+
+    assert "visual changes require screenshot proof" in doc
+    assert "do not commit screenshots or binary media to Gitea backup" in doc
+    assert "CLI/verifiable changes must cite the exact command output, log path, or world-state proof" in doc
+    assert "no proof, no merge" in doc
+
+
+def test_readme_points_to_proof_standard() -> None:
+    readme = Path("README.md").read_text()
+
+    assert "Proof Standard" in readme
+    assert "CONTRIBUTING.md" in readme