fix: include messaging dependency group in default Hermes build

Adds discord.py, python-telegram-bot, slack-bolt, and other messaging
platform libraries to the default Hermes derivation. This is required
for Discord/Slack/Telegram channel integration in k3s-deployed agents.

The [all] extras group stopped including messaging on 2026-05-12
(switched to lazy-install), but our Nix-built container images have
no pip available for runtime lazy installation.

Closes #2

nix/packages.nix

@@ -4,9 +4,10 @@
     let
       pkgs = inputs.nixpkgs.legacyPackages.${system};
     in {
-      default = inputs.hermes-agent.packages.${system}.default;
-
-      speckl-config = pkgs.writeTextDir "speckl-config.yaml" ''
+      # Hermes agent with messaging platform support (Discord, Slack, Telegram)
+      default = inputs.hermes-agent.packages.${system}.default.override {
+        extraDependencyGroups = [ "messaging" ];
+      }; = pkgs.writeTextDir "speckl-config.yaml" ''
         # Speckl SME — generated config
         # Override via services.hermes-speckl.settings
         model: