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/code/cli.md

@@ -297,6 +297,8 @@ helper a node calls into is reported as that node changing (`train: updated
 (flow, fit)`) and re-runs it, while editing something the node never reaches
 is left alone.
 
+Uploading node code takes a superuser's token; anyone else's sync is refused.
+
 A sync that changed nothing says `unchanged`, which is the answer worth
 having. `published` and `draft` are said only when there was something to
 publish or a draft was genuinely left behind. An engine too old to store what