nixos: Use codex-cli-nix and claude-code-nix flakes with cachix

Switch from manual version overrides to dedicated flakes:
- github:sadjow/codex-cli-nix for codex
- github:sadjow/claude-code-nix for claude-code

Added cachix substituters and keys for pre-built binaries.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nixos/nix.nix

@@ -33,11 +33,15 @@
           "https://cache.nixos.org"
           "https://cuda-maintainers.cachix.org"
           "https://ai.cachix.org"
+          "https://codex-cli.cachix.org"
+          "https://claude-code.cachix.org"
         ];
         trusted-public-keys = [
           "cache.railbird.ai:KhnvcouxtIU2zxUcjJsm4bUK3o1S3p8xMf9qfZGF7/A="
           "cuda-maintainers.cachix.org-1:0dq3bujKpuEPMCX6U4WylrUDZ9JyUG0VpVZa7CNfq5E="
           "ai.cachix.org-1:N9dzRK+alWwoKXQlnn0H6aUx0lU/mspIoz8hMvGvbbc="
+          "codex-cli.cachix.org-1:1Br3H1hHoRYG22n//cGKJOk3cQXgYobUel6O8DgSing="
+          "claude-code.cachix.org-1:YeXf2aNu7UTX8Vwrze0za1WEDS+4DuI2kVeWEE4fsRk="
         ];
         nix-path = nixPath;
       };
@@ -51,6 +55,11 @@
       # (import ./nvidia-container-toolkit-overlay.nix)
       (import ./runc-overlay.nix)
       (import ./overlay.nix)
+      # Use codex and claude-code from dedicated flakes with cachix
+      (final: prev: {
+        codex = inputs.codex-cli-nix.packages.${prev.stdenv.hostPlatform.system}.default;
+        claude-code = inputs.claude-code-nix.packages.${prev.stdenv.hostPlatform.system}.default;
+      })
     ] ++ (if config.imalison.nixOverlay.enable then [ inputs.nix.overlays.default ] else []);
 
     # Allow all the things