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/main.go

@@ -4,6 +4,7 @@
 //	go run ./cmd/gbrgames import export.json  # write games/<id>/ from GET /api/v1/admin/games
 //	go run ./cmd/gbrgames export -out p.json  # build the games_published payload CI sends to the panel
 //	go run ./cmd/gbrgames boottest-payload ID # build the deploy message the node-agent's BootTest runs
+//	go run ./cmd/gbrgames boot-test ID        # boot-test a game with the node-agent, as CI does (Steam cache)
 //	go run ./cmd/gbrgames inspect-image -game ID   # read the image's facts and check the template against them
 //	go run ./cmd/gbrgames first-boot -game ID      # start the server once, untouched; record its log and files
 //	go run ./cmd/gbrgames draft -name N -image I   # have an AI model write a new game's template from those facts
@@ -33,6 +34,8 @@ func main() {
 		err = runExport(os.Args[2:])
 	case "boottest-payload":
 		err = runBootTestPayload(os.Args[2:])
+	case "boot-test":
+		err = runBootTest(os.Args[2:])
 	case "inspect-image":
 		err = runInspectImage(os.Args[2:])
 	case "first-boot":
@@ -59,6 +62,6 @@ func main() {
 }
 
 func usage() {
-	fmt.Fprintln(os.Stderr, "usage: gbrgames validate [-games DIR] | import [-games DIR] [-all] EXPORT.json | export [-repo R] [-commit SHA] [-out FILE] | boottest-payload [-out FILE] ID | inspect-image [-game ID] [-json] [-no-pull] [IMAGE] | first-boot [-game ID | -image IMAGE] [-env K=V] [-max D] [-quiet D] [-out DIR] | draft -name NAME -image IMAGE [-steam-app ID] [-first-boot REPORT] [-provider ollama|gemini] [-model M] | research -name NAME [-notes N] [-out FILE] | build -name NAME -variant JSON|@FILE [-out FILE] | repair -id ID [-reason R] | eval [-ids all] [-out eval.md] | worker [-once]")
+	fmt.Fprintln(os.Stderr, "usage: gbrgames validate [-games DIR] | import [-games DIR] [-all] EXPORT.json | export [-repo R] [-commit SHA] [-out FILE] | boottest-payload [-out FILE] ID | boot-test ID | inspect-image [-game ID] [-json] [-no-pull] [IMAGE] | first-boot [-game ID | -image IMAGE] [-env K=V] [-max D] [-quiet D] [-out DIR] | draft -name NAME -image IMAGE [-steam-app ID] [-first-boot REPORT] [-provider ollama|gemini] [-model M] | research -name NAME [-notes N] [-out FILE] | build -name NAME -variant JSON|@FILE [-out FILE] | repair -id ID [-reason R] | eval [-ids all] [-out eval.md] | worker [-once]")
 	os.Exit(2)
 }