AGENTS.md: the shared rules name the Grbl sender, not one product

The "Project-wide rules" block is the same text in every repository
of the project, and this copy's "What ForgeFIRM is" said "controller
for LightBurn and other senders on TCP port 23", naming one sender as
the controller's counterpart. The project's term is "a Grbl sender",
the generic client of the controller on port 23, and no sender is
named as a synonym for it anywhere. The sentence now reads
"controller for a Grbl sender on TCP port 23", at the same wrap,
and nothing else in the block changes.

Proof: the block from "## Project-wide rules" to the end of the file
diffs empty against forgectrl's AGENTS.md; before the change the two
differed in that one line. pyflakes, the unit tests and the coverage
lint do not read AGENTS.md.

Acceptance: none. AGENTS.md is in no fingerprint and no catalog
test's covers; a text that guides contributors changes no behavior.

AGENTS.md

@@ -133,7 +133,7 @@ ForgeFIRM is open firmware for stock Glowforge lasers (Basic, Plus, Pro). It
 runs on the factory NXP i.MX6 control board with no hardware modification. It
 replaces the cloud-dependent factory userspace with an open Linux image that
 controls the machine locally. In GRBL mode the machine is a standard grblHAL
-controller for LightBurn and other senders on TCP port 23. Cloud mode runs the
+controller for a Grbl sender on TCP port 23. Cloud mode runs the
 factory experience through the Glowforge web service, and it is a deliberate,
 maintained feature. The stack is hardware validated: GRBL mode cuts real jobs,
 and cloud mode runs end to end.