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/dev-env/options.nix

@@ -121,8 +121,9 @@ in
       enable = mkEnableOption ''
         Bitcoin/Lightning regtest docker stack. Wraps an upstream
         fork of `lnbits/legend-regtest-enviroment` (LND + CLN +
-        Eclair + bitcoind + electrs). Implies a container engine —
-        the substantive pass will gate this on a containers feature
+        Eclair + bitcoind + electrs). Brought up via
+        `dev up --regtest`. Implies a container engine — the
+        substantive pass will gate this on a containers feature
       '';
 
       repoUrl = mkOption {
@@ -136,15 +137,6 @@ in
       };
     };
 
-    fakewallet = {
-      enable = mkEnableOption ''
-        FakeWallet dev mode helpers. Installs the symmetric `fakewallet`
-        wrapper script (a no-op for parity with `regtest up`) so the
-        dev experience is identical whether you're on the default
-        FakeWallet path or the full regtest stack
-      '';
-    };
-
     tmux = {
       enable = mkEnableOption "declarative tmux session launcher";