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>

cmd/gbrgames/boottest_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"time"
 	"encoding/json"
 	"errors"
 	"os"
@@ -121,3 +122,13 @@ func TestBootTestPayloadLeavesOutUnboundPorts(t *testing.T) {
 		t.Errorf("expectPorts %v", ports)
 	}
 }
+
+func TestSteamCacheStampAge(t *testing.T) {
+	now := time.Unix(1_800_000_000, 0)
+	if !stampWithin("1799990000\n", now, 24*time.Hour) {
+		t.Error("updated 10000s ago should be fresh")
+	}
+	if stampWithin("1700000000", now, 24*time.Hour) || stampWithin("", now, 24*time.Hour) || stampWithin("x", now, 24*time.Hour) {
+		t.Error("old, missing or garbled stamps are not fresh")
+	}
+}