feat(tui): /model and /setup slash commands with in-place CLI handoff

- hermes-ink: export `withInkSuspended()` + `useExternalProcess()` that
  pause/resume Ink around an arbitrary external process (built on the
  existing enterAlternateScreen/exitAlternateScreen plumbing)
- tui: `launchHermesCommand(args)` spawns the `hermes` binary with
  inherited stdio, with `HERMES_BIN` override for non-standard launches
- tui: `/model` and `/setup` slash commands invoke the CLI wizards
  in-place, then re-preflight `setup.status` and auto-start a session on
  success — no more exit-and-relaunch to finish first-run setup
- setup panel now advertises those slashes instead of only pointing
  users back at the shell

ui-tui/src/types/hermes-ink.d.ts

@@ -72,6 +72,9 @@ declare module '@hermes/ink' {
   export function render(node: React.ReactNode, options?: NodeJS.WriteStream | RenderOptions): Instance
 
   export function useApp(): { readonly exit: (error?: Error) => void }
+  export type RunExternalProcess = () => Promise<void>
+  export function useExternalProcess(): (run: RunExternalProcess) => Promise<void>
+  export function withInkSuspended(run: RunExternalProcess): Promise<void>
   export function useInput(handler: InputHandler, options?: { readonly isActive?: boolean }): void
   export function useSelection(): {
     readonly copySelection: () => string