feat(boot-test): CI boot tests use the Steam download cache, updated once a day

CI deployed every Steam game from scratch on each run, downloading the whole
game from Steam each time. scripts/boot-test.sh now runs the new
`gbrgames boot-test ID`, the builder's own deploy path: the same payload and
node-agent BootTest, but Steam games start from the cache volume
(gbr-steamcache-<app>, kept by RUNNER.md's prune).

The cache is updated at most once a day (a timestamp in the cache volume, not
copied into servers), so a repeat run within a day doesn't ask Steam for an
update at all; the node-agent's own install still checks the files and fetches
anything newer.

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

scripts/boot-test.sh

@@ -30,24 +30,14 @@ for id in "$@"; do
     echo "::endgroup::"
     continue
   fi
-  payload=$("$gbrgames" boottest-payload "$id")
+  # The node-agent's BootTest with the game's real payload. Steam games start from the runner's download
+  # cache (volumes gbr-steamcache-<app>, kept by RUNNER.md's prune), so Steam isn't asked for the whole
+  # game on every run
+  AGENT_IMAGE="$AGENT_IMAGE" "$gbrgames" boot-test "$id"
   code=$?
-  if [ $code -eq 3 ]; then
-    skipped+=("$id")
-    echo "::endgroup::"
-    continue
-  elif [ $code -ne 0 ]; then
-    failed+=("$id")
-    echo "::endgroup::"
-    continue
-  fi
-
-  docker run --rm -i \
-    -v /var/run/docker.sock:/var/run/docker.sock \
-    --entrypoint java "$AGENT_IMAGE" \
-    -Dloader.main=com.gbreed.gbr.nodeagent.BootTest -cp app.jar \
-    org.springframework.boot.loader.launch.PropertiesLauncher <<< "$payload"
-  if [ $? -eq 0 ]; then passed+=("$id"); else failed+=("$id"); fi
+  if [ $code -eq 3 ]; then skipped+=("$id")
+  elif [ $code -eq 0 ]; then passed+=("$id")
+  else failed+=("$id"); fi
   echo "::endgroup::"
 done