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/app/setupHandoff.ts

@@ -0,0 +1,54 @@
+import type { RunExternalProcess } from '@hermes/ink'
+
+import type { SetupStatusResponse } from '../gatewayTypes.js'
+import type { LaunchResult } from '../lib/externalCli.js'
+
+import type { SlashHandlerContext } from './interfaces.js'
+import { patchUiState } from './uiStore.js'
+
+export interface RunExternalSetupOptions {
+  args: string[]
+  ctx: Pick<SlashHandlerContext, 'gateway' | 'session' | 'transcript'>
+  done: string
+  launcher: (args: string[]) => Promise<LaunchResult>
+  suspend: (run: RunExternalProcess) => Promise<void>
+}
+
+export async function runExternalSetup({ args, ctx, done, launcher, suspend }: RunExternalSetupOptions) {
+  const { gateway, session, transcript } = ctx
+
+  transcript.sys(`launching \`hermes ${args.join(' ')}\`…`)
+  patchUiState({ status: 'setup running…' })
+
+  let result: LaunchResult = { code: null }
+
+  await suspend(async () => {
+    result = await launcher(args)
+  })
+
+  if (result.error) {
+    transcript.sys(`error launching hermes: ${result.error}`)
+    patchUiState({ status: 'setup required' })
+
+    return
+  }
+
+  if (result.code !== 0) {
+    transcript.sys(`hermes ${args[0]} exited with code ${result.code}`)
+    patchUiState({ status: 'setup required' })
+
+    return
+  }
+
+  const setup = await gateway.rpc<SetupStatusResponse>('setup.status', {})
+
+  if (setup?.provider_configured === false) {
+    transcript.sys('still no provider configured')
+    patchUiState({ status: 'setup required' })
+
+    return
+  }
+
+  transcript.sys(done)
+  session.newSession()
+}