setup: add ← Back option to Teams channel flow

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

Both Teams paths already had a brightSelect at the right place, so we
just extend each with a Back option — no new prompts:

- Existing-credentials path: Yes/No confirm becomes Yes/No/Back
- Fresh-setup path: the very first stepGate ("How did that go?") gets
  a 4th option. Subsequent stepGates keep the original 3 options so
  we never lose mid-flow state.

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

setup/auto.ts

@@ -462,7 +462,7 @@ async function main(): Promise<void> {
       } else if (channelChoice === 'signal') {
         await runSignalChannel(displayName!);
       } else if (channelChoice === 'teams') {
-        await runTeamsChannel(displayName!);
+        result = await runTeamsChannel(displayName!);
       } else if (channelChoice === 'slack') {
         result = await runSlackChannel(displayName!);
       } else if (channelChoice === 'imessage') {