feat(setup): wire Signal into the auto setup flow

`bash nanoclaw.sh` can now offer Signal as a channel choice, scan the
signal-cli link QR in the terminal, and wire up the first agent end to
end — mirroring the WhatsApp and Telegram flows.

Pieces:

- setup/add-signal.sh — non-interactive installer. Fetches
  src/channels/signal.ts + signal.test.ts from the channels branch,
  appends the self-registration import, installs qrcode (for the
  setup-flow QR render), and builds. Idempotent and standalone-runnable.

- setup/signal-auth.ts — step runner. Spawns `signal-cli link --name
  NanoClaw`, watches stdout for the `sgnl://linkdevice?…` (or legacy
  `tsdevice://`) URL, emits SIGNAL_AUTH_QR with it. On exit 0, runs
  `signal-cli -o json listAccounts` and reports the new account via
  SIGNAL_AUTH STATUS=success. Pre-check via listAccounts returns
  STATUS=skipped if an account is already linked.

- setup/channels/signal.ts — interactive driver. Probes for signal-cli
  (offering `brew install signal-cli` on macOS or linking GitHub
  releases on Linux if missing), runs add-signal.sh, renders each
  SIGNAL_AUTH_QR block as a terminal QR inside a clack spinner,
  persists SIGNAL_ACCOUNT to .env + data/env/env, restarts the
  service, then wires the first agent via init-first-agent.

- setup/index.ts: register `signal-auth` in the STEPS map.
- setup/auto.ts: add 'signal' to ChannelChoice, import the driver,
  add it to the channel picker (after WhatsApp, hint "needs signal-cli
  installed"), branch the dispatch, and map channelDmLabel.

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

setup/auto.ts

@@ -28,6 +28,7 @@ import k from 'kleur';
 
 import { runDiscordChannel } from './channels/discord.js';
 import { runIMessageChannel } from './channels/imessage.js';
+import { runSignalChannel } from './channels/signal.js';
 import { runSlackChannel } from './channels/slack.js';
 import { runTeamsChannel } from './channels/teams.js';
 import { runTelegramChannel } from './channels/telegram.js';
@@ -54,6 +55,7 @@ type ChannelChoice =
   | 'telegram'
   | 'discord'
   | 'whatsapp'
+  | 'signal'
   | 'teams'
   | 'slack'
   | 'imessage'
@@ -315,6 +317,8 @@ async function main(): Promise<void> {
       await runDiscordChannel(displayName!);
     } else if (channelChoice === 'whatsapp') {
       await runWhatsAppChannel(displayName!);
+    } else if (channelChoice === 'signal') {
+      await runSignalChannel(displayName!);
     } else if (channelChoice === 'teams') {
       await runTeamsChannel(displayName!);
     } else if (channelChoice === 'slack') {
@@ -442,6 +446,8 @@ function channelDmLabel(choice: ChannelChoice): string | null {
       return 'Discord DMs';
     case 'whatsapp':
       return 'WhatsApp';
+    case 'signal':
+      return 'Signal';
     case 'teams':
       return 'Teams';
     case 'imessage':
@@ -835,6 +841,11 @@ async function askChannelChoice(): Promise<ChannelChoice> {
         { value: 'telegram', label: 'Yes, connect Telegram', hint: 'recommended' },
         { value: 'discord', label: 'Yes, connect Discord' },
         { value: 'whatsapp', label: 'Yes, connect WhatsApp' },
+        {
+          value: 'signal',
+          label: 'Yes, connect Signal',
+          hint: 'needs signal-cli installed',
+        },
         {
           value: 'imessage',
           label: 'Yes, connect iMessage (experimental)',