chore: scaffold dev-env module (options + config + lib stubs)

default.nix composes the three sub-modules. options.nix declares the
public surface (projects, regtest, fakewallet, tmux) so consumers can
wire values today even though config.nix is empty. lib.nix reserves
dev-env-scoped helpers separate from the global lnbits-sensei.lib.

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

modules/dev-env/config.nix

@@ -0,0 +1,29 @@
+# lnbits-sensei dev-env — wire-up.
+#
+# Skeleton-only. The substantive pass will:
+#   - install the regtest.sh / fakewallet.sh wrappers on PATH
+#   - render a /etc/dev-env/config.sh consumed by the loose bash
+#     helpers (worktree nav, upstream-PR helper)
+#   - emit systemd.user units for any long-running pieces
+#   - hook into config.lnbits-sensei.git.remotes to drive the
+#     bootstrap script's remote reconciliation
+#
+# Empty body for now so the module composes cleanly.
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  inherit (lib) mkIf;
+  cfg = config.lnbits-sensei.devEnv;
+in
+{
+  config = mkIf cfg.enable {
+    # TODO(skeleton): wire scripts, systemd units, and the
+    # /etc/dev-env/config.sh render here. See omnixy
+    # modules/dev-env/config.nix for the reference shape.
+  };
+}