Add Signal Channel

Adds Signal support via a native adapter that speaks JSON-RPC to a signal-cli daemon — no Chat SDK bridge, only Node.js builtins. NanoClaw links to Signal as a secondary device on your existing phone: no new number, no bot API. Your assistant sends and receives as the number on the phone that scans the link.

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1. Install signal-cli

NanoClaw talks to Signal through signal-cli, which has no bot API of its own. Install it if it isn't on PATH yet — Homebrew on macOS, the native release binary on Linux (neither needs Java). If it's already installed this is a no-op:

command -v signal-cli >/dev/null 2>&1 || bash setup/install-signal-cli.sh