Make changing engine-run code superuser-only

Writing node source, installing modules and any flow save that adds,
retypes or repoints a code node (Function, Command), edits a Command
node's settings or changes a node's resources.env now answer 403 for
everyone but a superuser. MCP tools and the assistant reach the same
routes, so they inherit the gate. Node types report runs_code.

The editor shows code read-only with a note, disables a Command node's
settings, leaves code nodes and reusable nodes out of the palette and
paste, and the Modules page offers no install.

backend/tests/mcp/test_mcp_http.py

@@ -1,6 +1,7 @@
 """The MCP endpoint: who it lets in, and what happens once they are in."""
 
 import asyncio
+import json
 import uuid
 from collections.abc import Awaitable, Callable, Generator
 from datetime import timedelta
@@ -11,6 +12,7 @@ import pytest
 from fastapi.testclient import TestClient
 from sqlmodel import Session
 
+from fluksio.api.deps import CODE_IS_SUPERUSERS
 from fluksio.core import security
 from fluksio.core.config import settings
 from fluksio.main import app
@@ -170,3 +172,31 @@ def test_revoking_an_agent_stops_the_token_it_already_holds(
     before, after = over_mcp(block)
     assert before.status_code == 200
     assert after.status_code == 401
+
+
+def test_an_agent_cannot_write_code_its_person_may_not(
+    over_mcp,
+    client: TestClient,
+    normal_user_token_headers: dict[str, str],
+    agent: uuid.UUID,
+) -> None:
+    """The tools call the same routes, so an agent inherits the code gate."""
+    me = client.get(
+        f"{settings.API_V1_STR}/users/me", headers=normal_user_token_headers
+    ).json()
+    assert me["is_superuser"] is False
+    token = mcp_token(uuid.UUID(me["id"]), agent)
+
+    async def block(http: httpx.AsyncClient) -> httpx.Response:
+        return await http.post(
+            "/mcp",
+            **rpc(
+                token,
+                "tools/call",
+                name="save_node_source",
+                arguments={"name": "any", "node_id": "fn", "code": "import os"},
+            ),
+        )
+
+    answer = json.loads(over_mcp(block).json()["result"]["content"][0]["text"])
+    assert answer == {"error": CODE_IS_SUPERUSERS, "status": 403}