refactor: collapse fakewallet/regtest wrappers into single dev CLI

Replaces the two parallel scripts (fakewallet.sh, regtest.sh) with one
modules/dev-env/scripts/dev.sh — `dev up [--fakewallet|--regtest]`,
plus `down|logs|shell`. Default mode is fakewallet (no docker, no
chains, instant), matching what the prior scaffold did with two scripts
but giving consumers one command and one verb-set to learn.

Drops the now-redundant `lnbits.backend` enum and `features.fakewallet`
option from core.nix. Backend selection is the dev CLI's runtime
concern; a NixOS-level option would be a second knob that can disagree
with the CLI flag at runtime. `lnbits.{host,port}` stay (bind addr,
useful to docs and any later service path). `features.regtest` stays
(gates docker engine installation — consumers who'll never use the
regtest mode shouldn't pay for the container engine).

Strips modules/lnbits.nix entirely. The dev CLI runs lnbits ad-hoc; if
a NixOS-managed lnbits service becomes a real ask later, re-add a
focused module then.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

modules/core.nix

@@ -49,37 +49,20 @@ in
 
       regtest = mkEnableOption ''
         Full Bitcoin/Lightning regtest docker stack (LND + CLN + Eclair
-        + bitcoind + electrs). Implies a container engine. See
-        modules/dev-env/regtest.nix and modules/dev-env/scripts/regtest.sh
-      '';
-
-      fakewallet = mkEnableOption ''
-        FakeWallet dev mode — no docker, no chains, nothing to start.
-        Default LNbits backend; this flag mostly controls dev-env
-        helpers and symmetry with the regtest workflow
+        + bitcoind + electrs). Implies a container engine — install
+        docker only if you actually plan to invoke `dev up --regtest`.
+        The default `dev up` (FakeWallet) needs none of this
       '';
     };
 
-    # --- LNbits service config ---
+    # --- LNbits bind addr ---
+    #
+    # Backend selection lives in the `dev` CLI (--fakewallet | --regtest),
+    # not as a NixOS option: the dev-env scaffold doesn't manage a
+    # systemd lnbits service, and we don't want two knobs (build-time
+    # vs runtime) that can disagree.
 
     lnbits = {
-      backend = mkOption {
-        type = types.enum [
-          "FakeWallet"
-          "LndRestWallet"
-          "CoreLightningWallet"
-          "EclairWallet"
-        ];
-        default = "FakeWallet";
-        description = ''
-          Which Lightning backend LNbits talks to. FakeWallet is the
-          default — no real chain, no docker, nothing to start. The
-          regtest dev mode (modules/dev-env/regtest.nix) flips this to
-          LndRestWallet or CoreLightningWallet at activation time.
-        '';
-        example = "LndRestWallet";
-      };
-
       port = mkOption {
         type = types.port;
         default = 5000;