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/packages/hermes-ink/src/ink/hooks/use-external-process.ts

@@ -0,0 +1,27 @@
+import { useCallback } from 'react'
+
+import instances from '../instances.js'
+
+export type RunExternalProcess = () => Promise<void>
+
+export async function withInkSuspended(run: RunExternalProcess): Promise<void> {
+  const ink = instances.get(process.stdout)
+
+  if (!ink) {
+    await run()
+
+    return
+  }
+
+  ink.enterAlternateScreen()
+
+  try {
+    await run()
+  } finally {
+    ink.exitAlternateScreen()
+  }
+}
+
+export function useExternalProcess(): (run: RunExternalProcess) => Promise<void> {
+  return useCallback((run: RunExternalProcess) => withInkSuspended(run), [])
+}