feat(channels): add signal-cli channel adapter

config/default.yaml

@@ -9,6 +9,17 @@ telegram:
   bot_token: ${FLYNN_TELEGRAM_TOKEN}
   allowed_chat_ids: []  # Add your Telegram chat ID
 
+# Optional: Signal via signal-cli
+# signal:
+#   account: "+15551234567"
+#   signal_cli_path: signal-cli
+#   allowed_numbers: []      # Empty = allow all DMs
+#   allowed_group_ids: []    # Empty = no groups
+#   require_mention: true
+#   mention_name: flynn
+#   poll_interval_ms: 5000
+#   send_timeout_ms: 15000
+
 server:
   # Tailscale Serve config (optional). Enable `serve: true` to expose the
   # gateway to your tailnet via `tailscale serve`.