feat(builder): SteamCMD first for Linux servers, and fall back to the next image

A draft built on a ready-made image whose files all live outside /server
(nodes keep only /server and /data) went round three times and ended as
WIP, although research had also verified the game's Steam app, which
SteamCMD installs straight into /server. The builder never tried it.

- research reads each Steam app's OS list. When it ships a Linux server,
  cm2network/steamcmd goes first in the variant's images and that variant
  is recommended. Windows-only servers keep images with Proton or Wine
  first. The research prompt says the same.
- build tries the variant's images in order: when no draft passes with one,
  the next image gets a fresh start. The first draft that boots wins; if
  none does, the PR gets the draft that got furthest, and its summary lists
  each image tried and how it went.

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

cmd/gbrgames/research.go

@@ -173,9 +173,10 @@ const researchPrompt = "You research how to host a game's dedicated server for G
 	"in Docker on Linux, as a non-root user, with a volume at /server. From the search results and pages given, list " +
 	"the ways this game's dedicated server can be run (variants): separate editions of the game with their own server, " +
 	"crossplay builds, or notably different server images. For each variant give:\n" +
-	"- images: Docker Hub images that run this server, best first (prefer images that run as a non-root user, and the " +
-	"plain SteamCMD route cm2network/steamcmd when the game ships a Linux dedicated server on Steam). Only images that " +
-	"appear in the pages.\n" +
+	"- images: how to run this server, best first. When the game has a dedicated server on Steam that runs on Linux, " +
+	"cm2network/steamcmd (the plain SteamCMD route, installing the server into /server) always comes first and that " +
+	"variant is the recommended one. Then Docker Hub images that appear in the pages and run as a non-root user " +
+	"(needed for servers that only exist for Windows).\n" +
 	"- steamAppId: the Steam app id of the DEDICATED SERVER (not the game itself), if installed via SteamCMD; else \"\".\n" +
 	"- startupHint: the command that starts the server from its install folder /server, with the launch options " +
 	"for name, port, world and save location exactly as the pages document them; \"\" if unknown.\n" +
@@ -299,13 +300,13 @@ func trimToJSON(answer string) string {
 // Steam apps must exist and be readable anonymously (what SteamCMD installs need). What fails is dropped
 // and noted; a variant left with nothing runnable can't be recommended.
 func verifyVariants(res *researchResult) {
-	var apps []string
+	var appIDs []string
 	for _, v := range res.Variants {
 		if v.SteamAppID != "" {
-			apps = append(apps, v.SteamAppID)
+			appIDs = append(appIDs, v.SteamAppID)
 		}
 	}
-	names := steamAppNames(apps)
+	apps := steamApps(appIDs)
 	anyRunnable := false
 	for i := range res.Variants {
 		v := &res.Variants[i]
@@ -324,10 +325,11 @@ func verifyVariants(res *researchResult) {
 		}
 		v.Images = images
 		if v.SteamAppID != "" {
-			if name, ok := names[v.SteamAppID]; ok {
-				v.Verified = append(v.Verified, fmt.Sprintf("Steam app %s is %q", v.SteamAppID, name))
-				if !hasImage(v.Images, "cm2network/steamcmd") {
-					v.Images = append(v.Images, "cm2network/steamcmd")
+			if app, ok := apps[v.SteamAppID]; ok {
+				v.Verified = append(v.Verified, fmt.Sprintf("Steam app %s is %q", v.SteamAppID, app.name))
+				v.Images = withSteamCMD(v.Images, app.linux)
+				if app.linux {
+					v.Verified = append(v.Verified, "it ships a Linux server, so SteamCMD (installing it into /server) is tried first")
 				}
 			} else {
 				v.Problems = append(v.Problems, "Steam app "+v.SteamAppID+" not found for anonymous SteamCMD")
@@ -346,44 +348,56 @@ func verifyVariants(res *researchResult) {
 		res.Reason = "unknown"
 		res.Message = "No variant has a Docker image or Steam app that could be confirmed. " + res.Message
 	}
-	// Exactly one recommendation, among the runnable ones
+	recommend(res.Variants)
+}
+
+// recommend marks exactly one variant recommended, among the runnable ones. A variant that installs a Linux
+// server with SteamCMD comes before the model's choice: it keeps everything under /server, as nodes need.
+func recommend(variants []variant) {
+	runnable := func(v variant) bool { return len(v.Images) > 0 }
+	steamFirst := func(v variant) bool { return runnable(v) && isSteamCMDImage(v.Images[0]) }
 	rec := -1
-	for i, v := range res.Variants {
-		if v.Recommended && rec < 0 && len(v.Images) > 0 {
-			rec = i
-		}
-		res.Variants[i].Recommended = false
-	}
-	if rec < 0 {
-		for i, v := range res.Variants {
-			if len(v.Images) > 0 {
+	for _, want := range []func(variant) bool{
+		func(v variant) bool { return v.Recommended && steamFirst(v) },
+		steamFirst,
+		func(v variant) bool { return v.Recommended && runnable(v) },
+		runnable,
+	} {
+		for i, v := range variants {
+			if rec < 0 && want(v) {
 				rec = i
-				break
 			}
 		}
 	}
-	if rec >= 0 {
-		res.Variants[rec].Recommended = true
+	for i := range variants {
+		variants[i].Recommended = i == rec
 	}
 }
 
-func hasImage(images []string, name string) bool {
-	for _, i := range images {
-		if i == name || strings.HasPrefix(i, name+":") {
-			return true
-		}
-	}
-	return false
+var (
+	appName  = regexp.MustCompile(`"common"\s*\{[^}]*?"name"\s+"([^"]+)"`)
+	appLinux = regexp.MustCompile(`"oslist"\s+"[^"]*linux`)
+)
+
+type steamApp struct {
+	name  string
+	linux bool // ships a Linux server: SteamCMD installs it straight into /server
 }
 
-var appName = regexp.MustCompile(`"common"\s*\{[^}]*?"name"\s+"([^"]+)"`)
+func parseAppInfo(out string) (steamApp, bool) {
+	m := appName.FindStringSubmatch(out)
+	if m == nil {
+		return steamApp{}, false
+	}
+	return steamApp{name: m[1], linux: appLinux.MatchString(out)}, true
+}
 
-// steamAppNames asks SteamCMD (anonymously, in its official image) for each app's info: an app that
+// steamApps asks SteamCMD (anonymously, in its official image) for each app's info: an app that
 // anonymous SteamCMD can't see can't be installed by the node-agent either.
-func steamAppNames(apps []string) map[string]string {
-	names := map[string]string{}
+func steamApps(apps []string) map[string]steamApp {
+	found := map[string]steamApp{}
 	if len(apps) == 0 {
-		return names
+		return found
 	}
 	for _, app := range apps {
 		if !regexp.MustCompile(`^\d+$`).MatchString(app) {
@@ -395,9 +409,25 @@ func steamAppNames(apps []string) map[string]string {
 			fmt.Printf("[research] steamcmd for app %s: %v\n", app, err)
 			continue
 		}
-		if m := appName.FindStringSubmatch(out); m != nil {
-			names[app] = m[1]
+		if info, ok := parseAppInfo(out); ok {
+			found[app] = info
 		}
 	}
-	return names
+	return found
+}
+
+// withSteamCMD adds the plain SteamCMD route to a variant's images. When the Steam app ships a Linux server
+// it goes first: it installs into /server, which nodes keep, while ready-made images often keep their
+// files elsewhere. For Windows-only servers it stays last, behind images that bring Proton or Wine.
+func withSteamCMD(images []string, linux bool) []string {
+	var others []string
+	for _, i := range images {
+		if !isSteamCMDImage(i) {
+			others = append(others, i)
+		}
+	}
+	if linux {
+		return append([]string{"cm2network/steamcmd"}, others...)
+	}
+	return append(others, "cm2network/steamcmd")
 }