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_test.go

@@ -39,3 +39,52 @@ func TestAppNameFromAppInfo(t *testing.T) {
 		t.Errorf("got %v", m)
 	}
 }
+
+func TestParseAppInfoFindsLinuxServers(t *testing.T) {
+	linux, ok := parseAppInfo(`"1000000"
+{
+	"common"
+	{
+		"name"		"Example Dedicated Server"
+		"oslist"		"windows,linux"
+	}
+}`)
+	if !ok || linux.name != "Example Dedicated Server" || !linux.linux {
+		t.Errorf("got %+v %v", linux, ok)
+	}
+	// A server that only exists for Windows (needs Proton or Wine)
+	windows, _ := parseAppInfo(`"common" { "name" "Example Server" "oslist" "windows" }`)
+	if windows.linux {
+		t.Error("windows-only app reported as linux")
+	}
+}
+
+func TestWithSteamCMDGoesFirstOnlyForLinuxServers(t *testing.T) {
+	images := []string{"someone/example-server", "cm2network/steamcmd", "other/example"}
+	got := withSteamCMD(images, true)
+	if len(got) != 3 || got[0] != "cm2network/steamcmd" || got[1] != "someone/example-server" {
+		t.Errorf("linux: %v", got)
+	}
+	got = withSteamCMD([]string{"someone/proton-example"}, false)
+	if len(got) != 2 || got[1] != "cm2network/steamcmd" {
+		t.Errorf("windows-only: %v", got)
+	}
+}
+
+func TestRecommendPrefersTheSteamCMDVariant(t *testing.T) {
+	v := []variant{
+		{ID: "image", Recommended: true, Images: []string{"someone/example-server"}},
+		{ID: "steam", Images: []string{"cm2network/steamcmd", "someone/example-server"}},
+		{ID: "broken", Recommended: true},
+	}
+	recommend(v)
+	if v[0].Recommended || !v[1].Recommended || v[2].Recommended {
+		t.Errorf("got %+v", v)
+	}
+	// Without a SteamCMD variant, the model's runnable choice stands
+	v = []variant{{ID: "a", Images: []string{"x/a"}}, {ID: "b", Recommended: true, Images: []string{"x/b"}}}
+	recommend(v)
+	if v[0].Recommended || !v[1].Recommended {
+		t.Errorf("got %+v", v)
+	}
+}