feat(work): install gcx on the work profile

gcx is the Grafana Cloud CLI (dashboards, datasources, SLOs, synthetic
monitoring, alerts), used against the Citrix Grafana stack.

Pull it from nixpkgs-unstable via the existing overlay rather than the pinned
channel: 26.05 ships 0.2.14, which predates the stacks/contexts configuration
model and the agento11y commands, so the current tooling and docs do not apply
to it.

flake.nix

@@ -84,7 +84,9 @@
     flake-parts.lib.mkFlake { inherit inputs; } (
       { lib, ... }:
       let
-        # claude-code tracks nixpkgs-unstable regardless of the pinned nixpkgs.
+        # These track nixpkgs-unstable regardless of the pinned nixpkgs.
+        # gcx: 26.05 ships 0.2.14, which predates the stacks/contexts config
+        # model and the agento11y commands the tooling expects.
         overlays = [
           (_final: prev: {
             inherit
@@ -93,6 +95,7 @@
                 config.allowUnfree = true;
               })
               claude-code
+              gcx
               ;
           })
           # commitizen 4.13.9's regression test for the invalid-command error