fix(builder): drafts don't use CI secrets, and a missing one is fed back

The second Valheim build stopped before its boot test: told that
test.yaml's password "looks like a credential; map it under 'secrets'",
the model mapped it to a CI secret nobody created, and boottest-payload's
missing-secret exit ended the build.

- The validator now says to remove a test password first (boot tests fill
  password settings with a throwaway value); secrets only when the server
  can't start without a real one.
- Drafts with test.yaml 'secrets' fail the draft checks.
- build turns boottest-payload's missing-secret exit into feedback for the
  model instead of an error.

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

cmd/gbrgames/build.go

@@ -235,9 +235,18 @@ func bootCheck(gamesDir, id, agentImage string, wantOutside bool) (problems, ver
 	if err != nil {
 		return nil, nil, err
 	}
-	payload, err := exec.Command(self, "boottest-payload", "-games", gamesDir, id).Output()
+	payloadCmd := exec.Command(self, "boottest-payload", "-games", gamesDir, id)
+	var stderr bytes.Buffer
+	payloadCmd.Stderr = &stderr
+	payload, err := payloadCmd.Output()
 	if err != nil {
-		return nil, nil, fmt.Errorf("boottest-payload: %w", err)
+		var exitErr *exec.ExitError
+		if errors.As(err, &exitErr) && exitErr.ExitCode() == exitMissingSecret {
+			// The builder has no CI secrets: that's the template's problem to fix, not a reason to stop
+			return []string{"the boot test can't run: " + strings.TrimSpace(stderr.String()) + ". Remove 'secrets' from " +
+				"test.yaml; boot tests fill password settings with a throwaway value"}, nil, nil
+		}
+		return nil, nil, fmt.Errorf("boottest-payload: %v: %s", err, strings.TrimSpace(stderr.String()))
 	}
 	if out, err := runCmd("docker", "pull", "-q", agentImage); err != nil {
 		return nil, nil, fmt.Errorf("docker pull %s: %v: %s", agentImage, err, out)