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.

docs/reference/node-types.md

@@ -244,6 +244,7 @@ The cheapest fix for a sensor that reports every second and changes every hour.
 ### Command
 
 **`exec`** — run a command in the engine's container and read its output.
+Adding one or changing its settings is a superuser's, since the command is code.
 
 | Setting | Default | Notes |
 |---|---|---|