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/fr.json

@@ -348,6 +348,14 @@
     "emptyContentHint": "Aucun contenu pour le moment. Cliquez sur « Éditer » pour commencer.",
     "loadingSkill": "Chargement de la Skill...",
     "emptyNoAgents": "Aucun agent disponible.",
+    "noSelectionHint": "Sélectionnez un Skill à gauche ou cliquez sur « Nouveau Skill » pour en créer un.",
+    "scope": {
+      "global": "Global",
+      "folder": "Dossier",
+      "selectFolderPlaceholder": "Sélectionner un dossier",
+      "noFolders": "Aucun dossier trouvé",
+      "pickFolderHint": "Sélectionnez un dossier pour afficher ses Skills."
+    },
     "actions": {
       "preview": "Aperçu",
       "edit": "Éditer",