docs: add NixOS declarative plugin installation to nix-setup, plugins, and build-a-plugin guides

- nix-setup.md: new Plugins section with extraPlugins/extraPythonPackages
  examples, overlay usage, collision checking note, options reference rows
- plugins.md: Nix row in discovery table, NixOS declarative plugins section
- build-a-hermes-plugin.md: Distribute for NixOS section after pip section

website/docs/guides/build-a-hermes-plugin.md

@@ -633,6 +633,43 @@ pip install hermes-plugin-calculator
 # Plugin auto-discovered on next hermes startup
 ```
 
+### Distribute for NixOS
+
+NixOS users can install your plugin declaratively if you provide a `pyproject.toml` with entry points:
+
+**Entry-point plugins** (recommended for distribution):
+```nix
+# User's configuration.nix
+services.hermes-agent.extraPythonPackages = [
+  (pkgs.python312Packages.buildPythonPackage {
+    pname = "my-plugin";
+    version = "1.0.0";
+    src = pkgs.fetchFromGitHub {
+      owner = "you";
+      repo = "hermes-my-plugin";
+      rev = "v1.0.0";
+      hash = "sha256-...";  # nix-prefetch-url --unpack
+    };
+    format = "pyproject";
+    build-system = [ pkgs.python312Packages.setuptools ];
+  })
+];
+```
+
+**Directory plugins** (no `pyproject.toml` needed):
+```nix
+services.hermes-agent.extraPlugins = [
+  (pkgs.fetchFromGitHub {
+    owner = "you";
+    repo = "hermes-my-plugin";
+    rev = "v1.0.0";
+    hash = "sha256-...";
+  })
+];
+```
+
+See the [Nix Setup guide](/docs/getting-started/nix-setup#plugins) for complete documentation including overlay usage and collision checking.
+
 ## Common mistakes
 
 **Handler doesn't return JSON string:**