feat(nix): add extraDependencyGroups for sealed venv extras (#21817)

Expose the dependency-groups parameter from python.nix through
hermes-agent.nix and the NixOS module, allowing users to opt into
pyproject.toml optional extras (e.g. hindsight, voice, matrix) that
are resolved by uv inside the sealed venv.

Unlike extraPythonPackages (which appends to PYTHONPATH and requires
collision checking), extraDependencyGroups resolves the full dependency
graph in a single uv pass — no PYTHONPATH patching, no version
conflicts, no collision risk.

When to use which:
- extraDependencyGroups: enable a pyproject.toml optional extra
- extraPythonPackages: add an external Python plugin not in pyproject.toml

Usage:
  services.hermes-agent.extraDependencyGroups = [ "hindsight" ];

Or via overlay:
  pkgs.hermes-agent.override { extraDependencyGroups = [ "hindsight" ]; }

Refs: #8873, #9194

nix/hermes-agent.nix

@@ -1,7 +1,9 @@
 # nix/hermes-agent.nix — Overridable Hermes Agent package
 #
 # callPackage auto-wires nixpkgs args; flake inputs are passed explicitly.
-# Users override via: pkgs.hermes-agent.override { extraPythonPackages = [...]; }
+# Users override via:
+#   pkgs.hermes-agent.override { extraPythonPackages = [...]; }
+#   pkgs.hermes-agent.override { extraDependencyGroups = [ "hindsight" ]; }
 {
   lib,
   stdenv,
@@ -25,11 +27,13 @@
   rev ? null,
   # Overridable parameters
   extraPythonPackages ? [ ],
+  extraDependencyGroups ? [ ],
 }:
 let
   nodejs = nodejs_22;
   hermesVenv = callPackage ./python.nix {
     inherit uv2nix pyproject-nix pyproject-build-systems;
+    dependency-groups = [ "all" ] ++ extraDependencyGroups;
   };
 
   hermesNpmLib = callPackage ./lib.nix {