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/index.ts

@@ -16,6 +16,7 @@ const STEPS: Record<
   register: () => import('./register.js'),
   groups: () => import('./groups.js'),
   'whatsapp-auth': () => import('./whatsapp-auth.js'),
+  'signal-auth': () => import('./signal-auth.js'),
   mounts: () => import('./mounts.js'),
   service: () => import('./service.js'),
   verify: () => import('./verify.js'),