Strip comments and trailing whitespace from grub.cfg to fit modifications

The modified config was 75 bytes larger than the original (5798 vs 5723),
causing the in-place write to be rejected. Strip comment lines and trailing
whitespace to reclaim space for the added auto-installer kernel parameters.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/image/isomod.go

@@ -114,6 +114,11 @@ func rewriteGrubConfig(original, answerURL string) string {
 	for _, line := range lines {
 		trimmed := strings.TrimSpace(line)
 
+		// Strip comments to reclaim space for added kernel parameters
+		if strings.HasPrefix(trimmed, "#") {
+			continue
+		}
+
 		if strings.HasPrefix(trimmed, "set timeout=") {
 			result = append(result, "set timeout=0")
 			continue
@@ -143,10 +148,26 @@ func rewriteGrubConfig(original, answerURL string) string {
 			}
 		}
 
+		line = strings.TrimRight(line, " \t")
 		result = append(result, line)
 	}
 
-	return strings.Join(result, "\n")
+	// Collapse consecutive blank lines
+	var collapsed []string
+	prevBlank := false
+	for _, line := range result {
+		if line == "" {
+			if !prevBlank {
+				collapsed = append(collapsed, line)
+			}
+			prevBlank = true
+		} else {
+			collapsed = append(collapsed, line)
+			prevBlank = false
+		}
+	}
+
+	return strings.Join(collapsed, "\n")
 }
 
 // findAllOccurrences searches the entire ISO file for all locations where