docs(tray): restore F4-C identity-signal intent comment

The T9 full-file replacement accidentally dropped the inline comment
documenting why the count label is conditional on _todayCount > 0
(F4-C UX rationale). No behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main/tray.ts

@@ -14,6 +14,7 @@ let _todayCount = 0;
 
 function buildMenu() {
   const items: MenuItemConstructorOptions[] = [];
+  // F4-C: count > 0 시 비활성 라벨로 정체성 신호 노출. count = 0 시 메뉴를 자연스럽게 시작.
   if (_todayCount > 0) {
     items.push({ label: `오늘 ${_todayCount}번 잡아둠`, enabled: false });
     items.push({ type: 'separator' });