feat(shardr): Add shardr package to the overlay

Decentralized LLM repository (Go): content-addressed model store with
BitTorrent-based sync and OpenAI-compatible serving via llama-server.
Upstream has no tagged source releases, so the package is pinned to
main HEAD (fab7fa8) with a nix-update script targeting the branch.
Builds both shardr and shardhive binaries.

packages/shardr/default.nix

@@ -0,0 +1,2 @@
+{ pkgs, ... }:
+pkgs.callPackage ./package.nix { }

packages/shardr/package.nix

@@ -0,0 +1,48 @@
+{
+  lib,
+  buildGoModule,
+  fetchFromGitHub,
+}:
+
+buildGoModule rec {
+  pname = "shardr";
+  # Pinned to a commit: upstream has no tagged source releases, only
+  # prebuilt "shardr-runner" bundles.
+  version = "0.1.0+unstable";
+
+  src = fetchFromGitHub {
+    owner = "Cyb3rDudu";
+    repo = "shardr";
+    rev = "fab7fa89bc8164181c5bcebd1f23a74825d11de1";
+    hash = "sha256-bbNvTU3rSlKD7Wr+Q+WGKKtIVUq6uzmv93uZq0gzO2U=";
+  };
+
+  vendorHash = "sha256-wI7oVOFEKDK2hr7MmJ/QY9Mqwgk6t24cBe8HSM/zpVo=";
+
+  subPackages = [
+    "cmd/shardr"
+    "cmd/shardhive"
+  ];
+
+  # Upstream tests assume a llama-server binary and a live shardhive socket.
+  doCheck = false;
+
+  passthru = {
+    category = "AI Inference";
+    updateScript = [
+      "nix-update"
+      "--flake"
+      ".#shardr"
+      "--version=branch=main"
+    ];
+  };
+
+  meta = {
+    description = "Decentralized LLM repository with sync-based distribution over a BitTorrent peer network";
+    homepage = "https://github.com/Cyb3rDudu/shardr";
+    changelog = "https://github.com/Cyb3rDudu/shardr/releases";
+    license = lib.licenses.asl20;
+    maintainers = [ ];
+    mainProgram = "shardr";
+  };
+}