setup: add ← Back option to Signal channel flow

Stacked on #2269 (back-nav scaffolding) plus the Telegram, Slack, and
Teams PRs. They share the same scaffolding file from #2269 — they
don't compile without it, so they have to stack.

Signal had no user-facing prompt before the install kicked off, so
there was nothing to attach a Back option to. This adds a brief "Set
up Signal" info card (what's about to happen, no new phone number
needed) followed by a Continue/Back brightSelect. The card serves
double duty — context for the install plus the Back gate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup/auto.ts

@@ -460,7 +460,7 @@ async function main(): Promise<void> {
       } else if (channelChoice === 'whatsapp') {
         result = await runWhatsAppChannel(displayName!);
       } else if (channelChoice === 'signal') {
-        await runSignalChannel(displayName!);
+        result = await runSignalChannel(displayName!);
       } else if (channelChoice === 'teams') {
         result = await runTeamsChannel(displayName!);
       } else if (channelChoice === 'slack') {

setup/channels/signal.ts

@@ -33,6 +33,8 @@ import k from 'kleur';
 
 import * as setupLog from '../logs.js';
 import { getLaunchdLabel, getSystemdUnit } from '../../src/install-slug.js';
+import { BACK_TO_CHANNEL_SELECTION, type ChannelFlowResult } from '../lib/back-nav.js';
+import { brightSelect } from '../lib/bright-select.js';
 import {
   type Block,
   type StepResult,
@@ -48,7 +50,33 @@ import { accentGreen, fmtDuration, note } from '../lib/theme.js';
 
 const DEFAULT_AGENT_NAME = 'Nano';
 
-export async function runSignalChannel(displayName: string): Promise<void> {
+export async function runSignalChannel(displayName: string): Promise<ChannelFlowResult> {
+  note(
+    [
+      "NanoClaw links to Signal as a *secondary* device on your existing",
+      "phone — no new number needed. Your assistant will send and receive",
+      "messages as the number on that phone.",
+      '',
+      "Here's what's about to happen:",
+      '',
+      '  1. Check that signal-cli is installed (we\'ll guide you if not)',
+      '  2. Install the Signal adapter',
+      '  3. Show a QR code — scan it from Signal → Settings → Linked Devices',
+      '  4. Wire your assistant and send a welcome message',
+    ].join('\n'),
+    'Set up Signal',
+  );
+
+  const proceed = ensureAnswer(await brightSelect<'continue' | 'back'>({
+    message: 'Ready to set up Signal?',
+    options: [
+      { value: 'continue', label: 'Continue' },
+      { value: 'back', label: '← Back to channel selection' },
+    ],
+    initialValue: 'continue',
+  }));
+  if (proceed === 'back') return BACK_TO_CHANNEL_SELECTION;
+
   await ensureSignalCli();
 
   const install = await runQuietChild(