feat(skills): support folder-scoped skills in agent sessions

- Thread workspace path through useAgentSkills so Codex $-autocomplete
  surfaces folder-local skills in addition to global ones; cache keyed
  by agent + workspace and invalidated per key on focus.
- Add Global/Folder scope tabs and a folder picker (sourced from the
  folder table via loadFolderHistory) to the Skills settings page.
  CRUD for skills now operates against the selected scope and folder.
- Default the settings right panel to a placeholder hint; the new-skill
  form only appears after clicking "New Skill" or selecting an existing
  skill. Search input is hidden in folder scope.
- Disable "New Skill" when folder scope has no folder chosen; show a
  pick-folder hint in the path preview for that state.
- Add scope/noSelectionHint/pickFolderHint strings across 10 locales.

src/i18n/messages/ja.json

@@ -348,6 +348,14 @@
     "emptyContentHint": "まだ内容がありません。「編集」をクリックして開始してください。",
     "loadingSkill": "Skillを読み込み中...",
     "emptyNoAgents": "利用可能なエージェントがありません。",
+    "noSelectionHint": "左側から Skill を選択するか、「新規 Skill」をクリックして作成してください。",
+    "scope": {
+      "global": "グローバル",
+      "folder": "フォルダ",
+      "selectFolderPlaceholder": "フォルダを選択",
+      "noFolders": "フォルダが見つかりません",
+      "pickFolderHint": "Skills を表示するフォルダを選択してください。"
+    },
     "actions": {
       "preview": "プレビュー",
       "edit": "編集",