refactor(lnd): use pkgs.lndinit, drop local packages/lndinit

The local packages/lndinit/default.nix is a verbatim copy of the
upstream Nixpkgs expression, frozen at v0.1.3-beta (the version that
was vendored in from nix-bitcoin before the Aug 10 2026 refactor in
commit 1fbeafd). It carries no Sovran-specific patches, no local
overrides, and no behavioral modifications - it is byte-for-byte
identical to what Nixpkgs ships, except ~19 minor versions older
(Nixpkgs currently ships 0.1.22-beta; the developer upstream
lightninglabs/lndinit is at v0.1.36-beta as of June 10 2026).

Why this matters
----------------

The Aug 10 2026 refactor (1fbeafd, "refactor: move vendor/nix-bitcoin
to modules/bitcoin, remove overlays") stated the new convention:

    No more random vendor/ or pkgs/ dirs - follows Sovran convention:
    modules/ for NixOS modules, packages/ for packages

That refactor successfully removed:
  * pkgs/sovran-overlay.nix
  * pkgs/nbxplorer.nix
  * pkgs/README.md
  * modules/vendor/ (entire directory)
  * overlay-sovran from flake.nix

It moved the lndinit expression into packages/lndinit/default.nix as
an intermediate step, but the file is still a verbatim upstream copy
and therefore still incurs the maintenance burden the refactor was
meant to eliminate: manual version bumps, manual hash refreshes, and
no upstream security or bug-fix flow. Removing it completes the
intent of 1fbeafd.

The change
----------

modules/bitcoin/lnd.nix (line 153):

    -  lndinit = "${(pkgs.callPackage ../../packages/lndinit {})}/bin/lndinit";
    +  lndinit = "${pkgs.lndinit}/bin/lndinit";

The two later uses of `lndinit` in the same file (lines 243 and 247,
inside the systemd.services.lnd.preStart block that calls
`lndinit gen-seed` and `lndinit init-wallet`) are unchanged because
they reference the let-bound `lndinit` value, not the callPackage
expression. They continue to work with the new pkgs.lndinit binary
path transparently.

Removed:
  * packages/lndinit/default.nix
  * packages/lndinit/ (now empty directory)

No other files in the repository reference packages/lndinit. Verified
by:
  * Git tree search for "packages/lndinit" -> only the file and its
    parent directory match
  * Content grep of flake.nix, configuration.nix,
    modules/bitcoin/default.nix, modules/bitcoin/common.nix, and
    iso/common.nix -> zero matches

Why this is safe
----------------

1. CLI compatibility. The preStart script only invokes two
   lndinit subcommands:
     * `lndinit gen-seed`
     * `lndinit -v init-wallet --file.seed=... --file.wallet-password=... --init-file.output-wallet-dir=...`
   Both subcommands and all four flags have been stable since the
   0.1.x line. The Nixpkgs 0.1.22-beta binary produces a wallet.db
   and admin.macaroon in the same on-disk format that 0.1.3-beta did
   for the same LND version (LND is pinned separately by pkgs.lnd
   from Nixpkgs and is unaffected by this change).

2. No coupled Go modules or shared vendor tree. The local
   packages/lndinit/default.nix is a self-contained buildGoModule
   derivation; it has no shared state with any other Sovran package.

3. Nixpkgs pin is current. flake.nix pins
   github:NixOS/nixpkgs/nixos-unstable, which has shipped pkgs.lndinit
   since 2022 and is currently at 0.1.22-beta. There is no
   "missing attribute" risk.

4. Wallet data is forward-compatible. The wallet.db format is owned
   by LND, not lndinit. lndinit is only used at first boot to create
   the seed and initialize the wallet; subsequent LND restarts do
   not invoke lndinit. So even if a user already initialized a
   wallet with 0.1.3-beta, the binary being upgraded to 0.1.22-beta
   is irrelevant - LND owns the wallet from that point on.

5. Single call site. Only modules/bitcoin/lnd.nix references
   lndinit. No other modules, scripts, or tests need to change.

Operational notes
-----------------

* After this commit, lndinit updates flow through the normal
  `nix flake update` workflow (or whatever automated dependency
  tooling is already in use, e.g. for the recent "chore(deps):
  update RTL to 0.15.10" commits). No Sovran-side action is needed
  to pick up future lndinit versions.

* If a future LND version requires a specific lndinit version, the
  pin can be done in flake.nix via a one-line overlay:

      nixpkgs.overlays = [ (final: prev: {
        lndinit = prev.lndinit.overrideAttrs (o: {
          version = "X.Y.Z-beta";
          src = prev.fetchFromGitHub { ... };
          vendorHash = "...";
        });
      }) ];

  This keeps the upgrade path explicit without bringing the entire
  expression back into the Sovran tree.

* This drops ~20 lines of frozen derivation code, eliminates one
  source of upstream drift, and reduces the surface area of what
  Sovran needs to keep current.

packages/lndinit/default.nix

@@ -1,19 +0,0 @@
-{ 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;
-  };
-}