add contrib/shell.nix

Closes #12544

contrib/shell.nix

@@ -0,0 +1,33 @@
+# Environment containing all dependencies needed for
+# - building fish,
+# - building documentation,
+# - running all tests,
+# - formatting and checking lints.
+#
+# enter interactive bash shell:
+#   nix-shell contrib/shell.nix
+#
+# using system nixpkgs (otherwise fetches pinned version):
+#   nix-shell contrib/shell.nix --arg pkgs 'import <nixpkgs> {}'
+#
+# run single command:
+#   nix-shell contrib/shell --run "cargo xtask check"
+{ pkgs ? (import (builtins.fetchTarball {
+  url = "https://github.com/NixOS/nixpkgs/archive/nixos-25.11.tar.gz";
+  sha256 = "1ia5kjykm9xmrpwbzhbaf4cpwi3yaxr7shl6amj8dajvgbyh2yh4";
+}) { }), ... }:
+pkgs.mkShell {
+  buildInputs = [
+    (pkgs.python3.withPackages (pyPkgs: [ pyPkgs.pexpect ]))
+    pkgs.cargo
+    pkgs.clippy
+    pkgs.cmake
+    pkgs.gettext
+    pkgs.pcre2
+    pkgs.procps # tests use pgrep/pkill
+    pkgs.ruff
+    pkgs.rustc
+    pkgs.rustfmt
+    pkgs.sphinx
+  ];
+}