feat: add rtk as custom Nix derivation

Build rtk from source (not yet in nixpkgs) using rustPlatform.buildRustPackage.
Custom derivations extracted to pkgs/ for single responsibility.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkgs/rtk.nix

@@ -0,0 +1,15 @@
+{ rustPlatform, fetchFromGitHub, pkg-config, sqlite, lib }:
+rustPlatform.buildRustPackage {
+  pname = "rtk";
+  version = "0.22.2";
+  src = fetchFromGitHub {
+    owner = "rtk-ai";
+    repo = "rtk";
+    rev = "5b597002dcd99029cb9c0da9b6d38b44021bdb3a";
+    hash = "sha256-a/+5w1cU/sSGsIdu/9QT1lJlURF669FxMcTiR81makE=";
+  };
+  cargoHash = "sha256-lgmgorgT/KDSyzEcE33qkPF4f/3LJbAzEH0s9thTohE=";
+  doCheck = false; # tests require filesystem access (blocked by Nix sandbox)
+  nativeBuildInputs = [ pkg-config ];
+  buildInputs = [ sqlite ];
+}