Nix flake with a dev shell for everything

nix develop: Go, gopls, shellcheck, make, curl, jq, Python, Node.js,
oxipng and ImageMagick; on Linux also Ghostty, Xvfb, xdotool, DejaVu fonts
and Chromium for the README screenshots and the webOS app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dev/screenshots/take.sh

@@ -5,7 +5,7 @@
 #
 # Needs: go, python3, ghostty, xvfb-run (Xvfb), xdotool, ImageMagick 7,
 # fontconfig with DejaVu fonts; oxipng optional. On NixOS:
-#   nix-shell -p xvfb-run xorg-server xdotool imagemagick oxipng --run dev/screenshots/take.sh
+#   nix develop --command dev/screenshots/take.sh
 set -euo pipefail
 here=$(cd "$(dirname "$0")" && pwd)
 repo=$(cd "$here/../.." && pwd)