Fix panic on long directory paths in MultiTree and RefsTree

strings.Repeat panicked with negative count when the directory path
was longer than the default box width (60 chars). Added the same
width guard that Tree already had.

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

Author: JordanCoin

render/tree.go

@@ -202,6 +202,9 @@ func MultiTree(docs []*parser.Document, dirName string) {
 
 	// Title in top border (like codemap)
 	titleLine := fmt.Sprintf(" %s/ ", dirName)
+	if len(titleLine) > innerWidth {
+		innerWidth = len(titleLine) + 4
+	}
 	padding := innerWidth - len(titleLine)
 	leftPad := padding / 2
 	rightPad := padding - leftPad
@@ -393,6 +396,9 @@ func RefsTree(docs []*parser.Document, dirName string) {
 	// Header
 	innerWidth := 60
 	titleLine := fmt.Sprintf(" %s/ ", dirName)
+	if len(titleLine) > innerWidth {
+		innerWidth = len(titleLine) + 4
+	}
 	padding := innerWidth - len(titleLine)
 	leftPad := padding / 2
 	rightPad := padding - leftPad