#13972: feat: add Gossip channel extension

## Summary Adds a new **Gossip** channel extension so OpenClaw can send and receive messages via Gossip alongside the existing channels. ## Details - Introduces the `extensions/gossip` plugin with channel wiring and configuration - Hooks Gossip into the standard routing and channel abstractions - Adds user-facing documentation for configuring and using the Gossip channel ## Testing - Verified local setup of the Gossip extension - Sent and received messages through Gossip to confirm end-to-end behavior - CI passes for core and extension packages <img width="1476" height="1114" alt="image" src="https://github.com/user-attachments/assets/7afd95a6-d424-4d07-8f05-93ac8a10183e" />