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/zh-CN.json

@@ -348,6 +348,14 @@
     "emptyContentHint": "暂无内容。点击“编辑”开始输入。",
     "loadingSkill": "正在加载 Skill...",
     "emptyNoAgents": "暂无可用 Agent。",
+    "noSelectionHint": "从左侧选择一个 Skill，或点击“新建 Skill”创建。",
+    "scope": {
+      "global": "全局",
+      "folder": "文件夹",
+      "selectFolderPlaceholder": "选择文件夹",
+      "noFolders": "未找到任何文件夹",
+      "pickFolderHint": "选择一个文件夹以查看其 Skills。"
+    },
     "actions": {
       "preview": "预览",
       "edit": "编辑",