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 { }