Channel message API

This page moved to Channel outbound API.

openclaw/plugin-sdk/channel-message and openclaw/plugin-sdk/channel-message-runtime remain deprecated compatibility subpaths for older plugins; both are thin aliases over the shared channel message core. New channel plugins should use openclaw/plugin-sdk/channel-outbound for message lifecycle, receipt, durable send, and live preview helpers instead of adding new helpers to the deprecated subpaths.

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.