chore: require DECK env for dev scripts; drop LAN details from docs

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

scripts/test.sh

@@ -2,7 +2,7 @@
 # Run the Python unit tests on the Steam Deck (its python3 matches the target runtime closely).
 set -euo pipefail
 cd "$(dirname "$0")/.."
-DECK="${DECK:-deck@192.168.1.50}"
+DECK="${DECK:?set DECK to the Steam Deck SSH target, e.g. DECK=deck@steamdeck.local}"
 tar -cf - py_modules tests | ssh "$DECK" '
   rm -rf /tmp/mfd-test && mkdir /tmp/mfd-test && tar -xf - -C /tmp/mfd-test &&
   cd /tmp/mfd-test && PYTHONPATH=py_modules python3 -m unittest discover -s tests -t . -v'