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/checks.nix

@@ -240,6 +240,27 @@ json.dump(sorted(leaf_paths(DEFAULT_CONFIG)), sys.stdout, indent=2)
           echo "ok" > $out/result
         '';
 
+        # Verify extraDependencyGroups passes through to python.nix
+        extra-dependency-groups = let
+          hermesWithGroups = hermes-agent.override {
+            extraDependencyGroups = [ "honcho" ];
+          };
+        in pkgs.runCommand "hermes-extra-dependency-groups" { } ''
+          set -e
+          echo "=== Checking extraDependencyGroups override evaluates ==="
+
+          # Eval-only: verify the override produces valid derivation paths
+          # without building the full venv (which is expensive and redundant
+          # since the mechanism is just list concatenation into python.nix).
+          echo "derivation: ${hermesWithGroups}"
+          echo "venv: ${hermesWithGroups.hermesVenv}"
+          echo "PASS: extraDependencyGroups override evaluates cleanly"
+
+          echo "=== All extraDependencyGroups checks passed ==="
+          mkdir -p $out
+          echo "ok" > $out/result
+        '';
+
         # ── Config merge + round-trip test ────────────────────────────────
         # Tests the merge script (Nix activation behavior) across 7
         # scenarios, then verifies Python's load_config() reads correctly.