feat(research): one option per way to run a game, SteamCMD recommended

A draft built on a ready-made image whose files all live outside /server
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 build picked among the variant's images on its own.

- research reads each Steam app's OS list. When it ships a Linux server,
  the SteamCMD route is the recommended option; for Windows-only servers,
  images that bring Proton or Wine come first. The research prompt says
  the same.
- A variant that can run on several images becomes one option per image
  ("... (SteamCMD)", "... (image x/y)"), so the admin picks exactly how the
  server runs.
- build uses exactly the chosen option and never switches to another; if it
  can't be made to work, the PR says so and another option can be picked.

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

cmd/gbrgames/build.go

@@ -93,26 +93,35 @@ type buildOptions struct {
 	agentImage                    string
 }
 
+// build drafts the template for exactly the option the admin chose: one image (research lists each way to
+// run a game as its own option). It never switches to another image; if the chosen one can't be made to
+// work, the result says so and the admin can pick a different option.
 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
 	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
+		res, err := buildWithImage(o, image)
+		if res != nil {
+			res.Summary = buildSummary(o, res)
 		}
+		return res, err
+	}
+	return nil, errors.New("the variant has no image to build from")
+}
+
+// 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 +129,12 @@ 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 := ""