fix(commands): truncate session label by rune count, not byte count

Prevents splitting multi-byte UTF-8 characters (CJK, emoji) when
truncating session preview text in /session list output.

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

pkg/commands/cmd_session.go

@@ -109,10 +109,11 @@ func sessionLabel(summary, preview string, maxLen int) string {
 		return "(empty)"
 	}
 	text = strings.ReplaceAll(text, "\n", " ")
-	if len(text) <= maxLen {
+	runes := []rune(text)
+	if len(runes) <= maxLen {
 		return text
 	}
-	return text[:maxLen] + "..."
+	return string(runes[:maxLen]) + "..."
 }
 
 // extractSessionTag returns the "#N" suffix from a session key, or "#1" for