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/lib/externalCli.ts

@@ -0,0 +1,16 @@
+import { spawn } from 'node:child_process'
+
+export interface LaunchResult {
+  code: null | number
+  error?: string
+}
+
+const resolveHermesBin = () => process.env.HERMES_BIN?.trim() || 'hermes'
+
+export const launchHermesCommand = (args: string[]): Promise<LaunchResult> =>
+  new Promise(resolve => {
+    const child = spawn(resolveHermesBin(), args, { stdio: 'inherit' })
+
+    child.on('error', err => resolve({ code: null, error: err.message }))
+    child.on('exit', code => resolve({ code }))
+  })