feat: add copy note content option and migrate AGENTS.md to CLAUDE.md

Add a "Copy note content" option to the note options menu that copies the
event's content field to clipboard. Merge AGENTS.md into CLAUDE.md with
enhanced i18n rules emphasizing append-only locale key ordering.

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

src/i18n/locales/ja.ts

@@ -678,6 +678,7 @@ export default {
     'No notes found': 'ノートが見つかりません',
     'Try again later or check your connection':
       '後でもう一度お試しいただくか、接続を確認してください',
-    'Hide indirect': '間接通知を非表示'
+    'Hide indirect': '間接通知を非表示',
+    'Copy note content': 'ノート内容をコピー'
   }
 }