setup: add ← Back option to Discord, WhatsApp, iMessage channel flows

Picking the wrong messaging channel during setup left users with no way
to bail out — they had to either complete the chosen flow or kill setup
and start over. This adds a Back option to the first prompt of three
channel sub-flows that share the same simple shape (one leading
brightSelect that's easy to extend).

Mechanics:
- New `setup/lib/back-nav.ts` exports a BACK_TO_CHANNEL_SELECTION
  sentinel and ChannelFlowResult type.
- `setup/auto.ts` wraps the channel dispatch in a while-loop; channels
  return BACK_TO_CHANNEL_SELECTION to bounce back to the chooser
  without restarting setup. Channels not yet wired return void and the
  loop exits after one pass, so the change is backwards compatible.
- Discord, WhatsApp, iMessage each add a `← Back to channel selection`
  option to their first prompt.

Telegram, Slack, Teams, and Signal will follow as separate PRs — they
each need a slightly different shape (extra prompt insertions, gating
inside multi-step flows, etc.) and are easier to review independently.

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

setup/channels/discord.ts

@@ -27,6 +27,7 @@ import * as p from '@clack/prompts';
 import k from 'kleur';
 
 import * as setupLog from '../logs.js';
+import { BACK_TO_CHANNEL_SELECTION, type ChannelFlowResult } from '../lib/back-nav.js';
 import { brightSelect } from '../lib/bright-select.js';
 import { confirmThenOpen, formatNoteLink } from '../lib/browser.js';
 import { askOperatorRole } from '../lib/role-prompt.js';
@@ -48,8 +49,10 @@ interface AppInfo {
   owner: { id: string; username: string } | null;
 }
 
-export async function runDiscordChannel(displayName: string): Promise<void> {
-  const hasBot = await askHasBotToken();
+export async function runDiscordChannel(displayName: string): Promise<ChannelFlowResult> {
+  const choice = await askHasBotToken();
+  if (choice === 'back') return BACK_TO_CHANNEL_SELECTION;
+  const hasBot = choice === 'yes';
   if (!hasBot) {
     await walkThroughBotCreation();
   }
@@ -142,17 +145,18 @@ export async function runDiscordChannel(displayName: string): Promise<void> {
   }
 }
 
-async function askHasBotToken(): Promise<boolean> {
+async function askHasBotToken(): Promise<'yes' | 'no' | 'back'> {
   const answer = ensureAnswer(
     await brightSelect({
       message: 'Do you already have a Discord bot?',
       options: [
         { value: 'yes', label: 'Yes, I have a bot token ready' },
         { value: 'no', label: "No, walk me through creating one" },
+        { value: 'back', label: '← Back to channel selection' },
       ],
     }),
   );
-  return answer === 'yes';
+  return answer as 'yes' | 'no' | 'back';
 }
 
 async function walkThroughBotCreation(): Promise<void> {