feat: add ACT+ECT over MCP demo with LangGraph agent

End-to-end PoC demonstrating Agent Context Token authorization and
Execution Context Token accountability over MCP tool calls, using a
LangGraph agent with ES256-signed JWT tokens and DAG verification.

demo/act-ect-mcp/tests/conftest.py

@@ -0,0 +1,17 @@
+import shutil
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture
+def tmp_keys_dir(tmp_path) -> Path:
+    d = tmp_path / "keys"
+    d.mkdir()
+    return d
+
+
+@pytest.fixture
+def identities(tmp_keys_dir):
+    from poc.keys import load_identities
+    return load_identities(tmp_keys_dir)