setup: add ← Back option to Telegram channel flow

Stacked on the back-nav scaffolding from the Discord/WhatsApp/iMessage
PR — depends on setup/lib/back-nav.ts and the auto.ts loop.

Telegram's "no existing token" path adds one extra prompt — a
brightSelect "Ready to paste your bot token?" between the BotFather
instructions and the token paste. Clack's p.password prompt doesn't
support menu options so we can't fold Back into the paste itself; the
cleanest fix is a separate gate immediately before. The "existing
token" path doesn't add noise — the Yes/No confirm becomes Yes/No/Back.

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

setup/auto.ts

@@ -454,7 +454,7 @@ async function main(): Promise<void> {
       }
       let result: void | typeof BACK_TO_CHANNEL_SELECTION;
       if (channelChoice === 'telegram') {
-        await runTelegramChannel(displayName!);
+        result = await runTelegramChannel(displayName!);
       } else if (channelChoice === 'discord') {
         result = await runDiscordChannel(displayName!);
       } else if (channelChoice === 'whatsapp') {