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

modules/bitcoin/versioning.nix

@@ -0,0 +1,12 @@
+{ config, lib, ... }:
+with lib;
+let
+  options.nix-bitcoin.configVersion = mkOption {
+    type = with types; nullOr str;
+    default = null;
+    description = "Vendored stub — no version migration needed.";
+  };
+in {
+  inherit options;
+  config = {};
+}