docs(demo): note path variable limitation in find_related query

CoordiNode does not yet support MATCH p=.../nodes(p) path variables.
Document why session isolation holds without it: all nodes are MERGE'd
with session scope so cross-session paths cannot form in practice.

Author: polaz

demo/notebooks/03_langgraph_agent.ipynb

@@ -269,6 +269,7 @@
     "def find_related(entity_name: str, depth: int = 1) -> str:\n",
     "    \"\"\"Find all entities reachable from entity_name within the given number of hops (max 3).\"\"\"\n",
     "    safe_depth = max(1, min(int(depth), 3))\n",
+    "    # Note: session constraint is on both endpoints (n, m). Constraining\n    # intermediate nodes via path variables (MATCH p=..., WHERE ALL(x IN nodes(p)...))\n    # is not yet supported by CoordiNode — planned for a future release.\n    # In practice, session isolation holds because all nodes are MERGE'd with\n    # their session scope, so cross-session paths cannot form.\n",
     "    rows = client.cypher(\n",
     "        f\"MATCH (n:Entity {{name: $name, session: $sess}})-[*1..{safe_depth}]->(m:Entity {{session: $sess}}) \"\n",
     "        \"RETURN DISTINCT m.name AS related LIMIT 20\",\n",