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/es.json

@@ -348,6 +348,14 @@
     "emptyContentHint": "Aún no hay contenido. Haz clic en \"Editar\" para empezar.",
     "loadingSkill": "Cargando Skill...",
     "emptyNoAgents": "No hay agentes disponibles.",
+    "noSelectionHint": "Selecciona un Skill a la izquierda o haz clic en \"Nuevo Skill\" para crear uno.",
+    "scope": {
+      "global": "Global",
+      "folder": "Carpeta",
+      "selectFolderPlaceholder": "Seleccionar una carpeta",
+      "noFolders": "No se encontraron carpetas",
+      "pickFolderHint": "Selecciona una carpeta para ver sus Skills."
+    },
     "actions": {
       "preview": "Vista previa",
       "edit": "Editar",