refactor: move vendor/nix-bitcoin to modules/bitcoin, remove overlays

- Move modules/vendor/nix-bitcoin/* -> modules/bitcoin/* (clean, Sovran-owned)
  * modules/bitcoin/default.nix imports the 6 tailored services
  * modules/bitcoin/common.nix bundles secrets/onion/lib
  * modules/bitcoin/stubs.nix kept minimal (no clightning)
  * packages/lndinit/default.nix replaces pkgs/sovran-overlay.nix
    (lnd.nix now uses pkgs.callPackage ../../packages/lndinit {})
- Remove pkgs/sovran-overlay.nix, pkgs/nbxplorer.nix, pkgs/README.md
  * No global overlay - lndinit is a normal package in packages/
- Remove modules/vendor/ entirely
- Update flake.nix: drop overlay-sovran, import ./modules/bitcoin
  instead of ./modules/vendor/nix-bitcoin/modules.nix
- No more random vendor/ or pkgs/ dirs - follows Sovran convention:
  modules/ for NixOS modules, packages/ for packages

packages/lndinit/default.nix

@@ -0,0 +1,19 @@
+{ lib, buildGoModule, fetchFromGitHub }:
+
+buildGoModule rec {
+  pname = "lndinit";
+  version = "0.1.3-beta";
+  src = fetchFromGitHub {
+    owner = "lightninglabs";
+    repo = pname;
+    rev = "v${version}";
+    sha256 = "sha256-sO1DpbppCurxr9g9nUl9Vx82FJK1mTcUw3rY1Fm1wEU=";
+  };
+  vendorHash = "sha256-El44BS5Bu0K/klMxkajciU/R6uqiXBMOiLN536QztbE=";
+  subPackages = [ "." ];
+  meta = with lib; {
+    description = "Wallet initializer for lnd (from nix-bitcoin)";
+    homepage = "https://github.com/lightninglabs/lndinit";
+    license = licenses.mit;
+  };
+}