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/components/chat/message-input.tsx

@@ -331,7 +331,10 @@ export function MessageInput({
   // set through ACP `availableCommands`, so injecting skills there would
   // be duplicate/extra UI noise — skip the skills fetch for them entirely.
   const skillAgentType = agentType === "codex" ? "codex" : null
-  const availableSkills = useAgentSkills(skillAgentType)
+  // Pass the working dir so we see both global skills and folder-scoped
+  // project skills (e.g. `{folder}/.codex/skills`). Without this, users
+  // only ever saw global skills in the `$` autocomplete.
+  const availableSkills = useAgentSkills(skillAgentType, defaultPath ?? null)
   // Expert skills are symlinked into the agent's skill directories, so they
   // also show up in `acp_list_agent_skills`. Strip them out — experts remain
   // reachable via the expert button, and the `$` list is skills-only.