Add iMessage

NanoClaw talks to iMessage through a single imessage channel with two pluggable backends:

• Local (this Mac) — the Chat SDK bridge over chat-adapter-imessage, reading this Mac's signed-in iMessage account (chat.db). macOS only; the Node binary needs Full Disk Access.
• Hosted iMessage (via photon.codes) — a native adapter over Photon's spectrum-ts gRPC stream. The hosted service owns the iMessage line, so there's no Mac relay, webhook, or public URL. Works on any OS, and a device-login flow provisions everything for you.

Both register the same imessage channel type; only one runs per install. NanoClaw doesn't ship channels in trunk — this skill copies the unified imessage adapter in from the channels branch. Full reference: docs/imessage.md.