S06-S08: Python spec, Go in-process leaf, tests and Forgejo CI

python/secure_messaging/__init__.py

@@ -0,0 +1,13 @@
+# Public package surface for the readable Python spec.
+from .signed_config import SignedConfig, UnsignedConfig  # signed wrapper load/save
+from .envelope import PassthroughEnvelope  # dest-in-clear, body ciphertext
+from .error_bundle import NetworkErrorBundle  # system-key + sender-only
+from .admin_history import AdminHistory  # DataCube-shaped append-only config log
+
+__all__ = [
+    "SignedConfig",
+    "UnsignedConfig",
+    "PassthroughEnvelope",
+    "NetworkErrorBundle",
+    "AdminHistory",
+]