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/lib/back-nav.ts

@@ -0,0 +1,17 @@
+/**
+ * Channel-flow back-navigation sentinel.
+ *
+ * Each `runXxxChannel(displayName)` in `setup/channels/` may return either
+ * `void` (sub-flow completed normally) or `BACK_TO_CHANNEL_SELECTION` to
+ * signal "the user picked '← Back to channel selection' on my first
+ * prompt; please re-run the channel chooser." `setup/auto.ts` catches
+ * that signal and loops back to `askChannelChoice()`.
+ *
+ * Back is only offered on the *first* interactive prompt of each channel
+ * sub-flow — once the user has answered something, they're committed
+ * (subsequent steps may have side effects like opening browsers, hitting
+ * APIs, or installing adapter packages, none of which are easily undone).
+ */
+export const BACK_TO_CHANNEL_SELECTION = Symbol('BACK_TO_CHANNEL_SELECTION');
+
+export type ChannelFlowResult = void | typeof BACK_TO_CHANNEL_SELECTION;