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/en.json

@@ -348,6 +348,14 @@
     "emptyContentHint": "No content yet. Click \"Edit\" to start.",
     "loadingSkill": "Loading skill...",
     "emptyNoAgents": "No available agent.",
+    "noSelectionHint": "Select a skill on the left, or click \"New Skill\" to create one.",
+    "scope": {
+      "global": "Global",
+      "folder": "Folder",
+      "selectFolderPlaceholder": "Select a folder",
+      "noFolders": "No folders found",
+      "pickFolderHint": "Select a folder to view its skills."
+    },
     "actions": {
       "preview": "Preview",
       "edit": "Edit",