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/build.go

@@ -27,6 +27,7 @@ type buildResult struct {
 	Verified []string `json:"verified"`
 	Problems []string `json:"problems"`
 	Summary  string   `json:"summary"`
+	Tried    []string `json:"tried,omitempty"` // each image tried, and how it went
 }
 
 const defaultAgentImage = "www.gitea.gbreed.com/gbreed-public/gbr-node-agent:latest"
@@ -93,26 +94,81 @@ type buildOptions struct {
 	agentImage                    string
 }
 
+// build tries the variant's images in order (research puts SteamCMD first when the game ships a Linux
+// server) and keeps the first draft that boots. A draft that can't be made to work with one image is set
+// aside and the next image gets a fresh start; if none works, the draft that got furthest is returned.
 func build(o buildOptions) (*buildResult, error) {
-	if len(o.variant.Images) == 0 {
-		return nil, errors.New("the variant has no image to build from")
-	}
-	res := &buildResult{GameID: o.id, Dir: filepath.Join(o.gamesDir, o.id)}
-	res.Verified = append(res.Verified, o.variant.Verified...)
-
-	// 1. First boot of each candidate image until one runs as a non-root user
-	var report *bootReport
+	var candidates []string
 	for _, image := range o.variant.Images {
-		steam := isSteamCMDImage(image) && o.variant.SteamAppID != ""
-		if isSteamCMDImage(image) && !steam {
+		if isSteamCMDImage(image) && o.variant.SteamAppID == "" {
 			continue // the SteamCMD image alone runs nothing
 		}
-		res.Image = image
-		if steam && o.variant.StartupHint == "" {
-			fmt.Printf("[build] no startup command known for Steam app %s; drafting without a first boot\n", o.variant.SteamAppID)
-			report = nil
-			break
+		candidates = append(candidates, image)
+	}
+	if len(candidates) == 0 {
+		return nil, errors.New("the variant has no image to build from")
+	}
+	dir := filepath.Join(o.gamesDir, o.id)
+	setAside := filepath.Join("firstboot-out", o.id+"-best-draft")
+	var best *buildResult
+	var tried []string
+	for i, image := range candidates {
+		if i > 0 {
+			fmt.Printf("[build] trying the next image: %s\n", image)
 		}
+		res, err := buildWithImage(o, image)
+		if err != nil {
+			return res, err
+		}
+		tried = append(tried, attemptNote(res))
+		if res.Passed {
+			res.Tried = tried
+			res.Summary = buildSummary(o, res)
+			return res, nil
+		}
+		if _, err := os.Stat(filepath.Join(dir, "game.yaml")); err == nil {
+			// This one got a draft written: keep it in case no later image does better
+			os.RemoveAll(setAside)
+			if err := os.Rename(dir, setAside); err != nil {
+				return res, err
+			}
+			best = res
+		}
+	}
+	if best == nil {
+		return &buildResult{GameID: o.id, Dir: dir, Tried: tried, Problems: []string{"no image of this variant could be drafted"}},
+			errors.New("no image of this variant could be drafted: " + strings.Join(tried, "; "))
+	}
+	if err := os.Rename(setAside, dir); err != nil {
+		return best, err
+	}
+	best.Tried = tried
+	best.Summary = buildSummary(o, best)
+	return best, nil
+}
+
+func attemptNote(res *buildResult) string {
+	switch {
+	case res.Passed:
+		return res.Image + ": booted"
+	case len(res.Problems) > 0:
+		return res.Image + ": " + firstLine(res.Problems[0])
+	}
+	return res.Image + ": did not boot"
+}
+
+// buildWithImage drafts the template for one image: first boot, draft, then boots of the draft with
+// failures going back to the model.
+func buildWithImage(o buildOptions, image string) (*buildResult, error) {
+	res := &buildResult{GameID: o.id, Dir: filepath.Join(o.gamesDir, o.id), Image: image}
+	res.Verified = append(res.Verified, o.variant.Verified...)
+	steam := isSteamCMDImage(image)
+
+	// 1. First boot of the image untouched: its log, files and settings are the draft's facts
+	var report *bootReport
+	if steam && o.variant.StartupHint == "" {
+		fmt.Printf("[build] no startup command known for Steam app %s; drafting without a first boot\n", o.variant.SteamAppID)
+	} else {
 		fmt.Printf("[build] first boot of %s\n", image)
 		r, err := firstBoot(o.id+"-probe", image, o.variant.SteamAppID, steam, o.variant.StartupHint)
 		if err != nil {
@@ -120,18 +176,12 @@ func build(o buildOptions) (*buildResult, error) {
 		}
 		if why := imageUnusable(r); why != "" {
 			fmt.Printf("[build] %s can't be used: %s\n", image, why)
-			res.Problems = append(res.Problems, why)
-			res.Image = ""
-			continue
+			res.Problems = []string{why}
+			return res, nil
 		}
 		report = r
 		res.Verified = append(res.Verified, fmt.Sprintf("first boot of %s: %s", image, r.Outcome))
-		break
 	}
-	if res.Image == "" {
-		return res, errors.New("no image of this variant runs as a non-root user")
-	}
-	res.Problems = nil
 
 	// 2. Draft until validation and the image checks pass
 	steamApp := ""
@@ -411,6 +461,12 @@ func buildSummary(o buildOptions, res *buildResult) string {
 		fmt.Fprintf(&b, ", Steam app %s", o.variant.SteamAppID)
 	}
 	fmt.Fprintf(&b, "\n- Model rounds: %d\n", res.Rounds)
+	if len(res.Tried) > 1 {
+		b.WriteString("\n**Images tried, in order**\n")
+		for _, t := range res.Tried {
+			fmt.Fprintf(&b, "- %s\n", firstLine(t))
+		}
+	}
 	if len(res.Verified) > 0 {
 		b.WriteString("\n**Checked for real**\n")
 		for _, v := range res.Verified {