Channel message API

This page moved to Channel outbound API.

OmeniaClaw/plugin-sdk/channel-message and OmeniaClaw/plugin-sdk/channel-message-runtime remain deprecated compatibility subpaths for older plugins. New channel plugins should use OmeniaClaw/plugin-sdk/channel-outbound for message lifecycle, receipt, durable send, and live preview helpers. The deprecated subpaths are thin aliases over the shared channel message core and the focused inbound/outbound SDK surfaces; do not add new helpers there.

Removal plan: keep these aliases through the external plugin migration window, then remove them in the next major SDK cleanup after callers have moved to channel-outbound.