fix(builder): boot drafts with the settings a new server really gets

The builder's boot test filled every blank password-like setting with a
throwaway value. A real deploy only fills x-generate fields, so a draft whose
server refuses to run with an empty join password passed the builder and CI,
and then shut down on its first real deploy.

The builder now boots drafts with boottest-payload -as-deployed: only
x-generate fields are filled, everything else exactly as the template's
defaults. A server that won't run with its own defaults fails the build, and
the model is told that empty settings stay empty, so the value must come from
a default or x-generate. CI's boot test is unchanged.

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

cmd/gbrgames/boottest_test.go

@@ -63,4 +63,20 @@ configSchema:
 	if c["MOTD"] != "gbr-boot-test on 25565" {
 		t.Errorf("placeholders not resolved: %v", c["MOTD"])
 	}
+
+	// As a new server gets it (what the AI builder boots): only x-generate is filled, a blank join password stays blank
+	if err := runBootTestPayload([]string{"-as-deployed", "-games", filepath.Join(dir, "games"), "-out", out, "demo"}); err != nil {
+		t.Fatal(err)
+	}
+	raw, _ = os.ReadFile(out)
+	payload.GameConfig = nil
+	if err := json.Unmarshal(raw, &payload); err != nil {
+		t.Fatal(err)
+	}
+	if s, _ := payload.GameConfig["RCON_PASSWORD"].(string); !strings.HasPrefix(s, "boot-") {
+		t.Errorf("as deployed: x-generate RCON_PASSWORD not filled: %v", payload.GameConfig["RCON_PASSWORD"])
+	}
+	if payload.GameConfig["ServerPassword"] != "" {
+		t.Errorf("as deployed: ServerPassword should stay empty, got %v", payload.GameConfig["ServerPassword"])
+	}
 }