docs/RUST.md: point at the this-machine-graphics skill

The GPU findings from 2026-09-08 would bite any project on this machine,
not just this one, so they are now a skill (AGENTS.md's own rule about
where a machine-wide lesson belongs). This section keeps the iris- and
port-specific half and names the skill for the rest.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/RUST.md

@@ -7501,6 +7501,13 @@ size.
 
 ### Venus went away for an hour, and nothing said so (2026-09-08)
 
+**The machine-wide version of this is the `this-machine-graphics`
+skill**, written the same day so an agent in any repo gets it: what this
+VM's GPU is, the probe that asks it what it supports, this outage's
+signature, the wgpu-on-Venus teardown crash, and the rule about never
+`.expect()`ing a `PRIMARY` adapter. What stays here is the part specific
+to iris and to this port.
+
 **It came back on its own and nothing in the host config changed.** Iris
 asked whether something had, since she still passes Venus as true. What
 happened, and what was ruled out, so the next occurrence is not