feat: add Garnix CI and binary cache

Add garnix.yaml to build all system configurations, dev shells, and
checks. Configure cache.garnix.io as a substituter so local builds
benefit from CI-cached artifacts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: stackptr

flake.nix

@@ -115,9 +115,11 @@
         nixConfig = {
           experimental-features = ["nix-command" "flakes"];
           extra-substituters = [
+            "https://cache.garnix.io"
             "https://stackptr.cachix.org"
           ];
           extra-trusted-public-keys = [
+            "cache.garnix.io:CTFPyKSLcx5RMJKfLo5EEPUObbA78b0YQ2DTCJXqr9g="
             "stackptr.cachix.org-1:5e2q7OxdRdAtvRmHTeogpgJKzQhbvFqNMmCMw71opZA="
           ];
         };

garnix.yaml

@@ -0,0 +1,7 @@
+builds:
+  include:
+    - "nixosConfigurations.*"
+    - "darwinConfigurations.*"
+    - "devShells.*.*"
+    - "checks.*.*"
+  exclude: []