feat(claude): add @AGENTS.md reference to dotfiles CLAUDE.md

The preprocessor picks up @AGENTS.md at load time, making AGENTS.md
the source of truth for agent instructions without an extra round trip.

Author: Castrozan

home/modules/claude/config.nix

@@ -54,6 +54,8 @@ let
   claudeGlobalSettingsJson = builtins.toJSON claudeGlobalSettings;
 
   claudeDotfilesRules = ''
+    @AGENTS.md
+
     # Dotfiles Repository
 
     Nix-based dotfiles repository managing system and home configurations across NixOS and standalone home-manager hosts. Use /dotfiles skill for repository patterns, directory organization, and anti-patterns. Use /rebuild to apply changes. Use /test for verification. Core behavior instructions are in the global CLAUDE.md — do not duplicate them here.