chore(nix->guix): remove flake.nix (Guix-only) (#275)

Removes flake.nix/flake.lock per the estate Nix ban (Guix sole pkg mgr).
Fresh from origin. 2026-06-24. (added guix.scm)

Author: hyperpolymath

flake.nix

@@ -0,0 +1,12 @@
+;; SPDX-License-Identifier: MPL-2.0
+;; Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+;;
+;; Guix development environment for echo-types. Replaces flake.nix (Guix-only policy).
+;; Usage: guix shell -D -f guix.scm
+(use-modules (guix packages) (guix build-system gnu))
+(package
+  (name "echo-types") (version "0.1.0") (source #f)
+  (build-system gnu-build-system)
+  (synopsis "echo-types") (description "echo-types — part of the hyperpolymath ecosystem.")
+  (home-page "https://github.com/hyperpolymath/echo-types")
+  (license ((@@ (guix licenses) license) "MPL-2.0" "https://github.com/hyperpolymath/palimpsest-license")))