feat: add kchat websocket client

[OpenCow42]

Summary

• add reusable kChat WebSocket helpers to liquid-potassium
• support Infomaniak Echo/Pusher subscriptions plus Mattermost WebSocket auth
• expose the client from createInfomaniakClient and the public package exports
• document the new kChat WebSocket API

Validation

• npm run ci
• OpenClaw temporary branch dependency smoke: npm test, npm run check
• Live OpenClaw smoke via SDK branch: sent and received the same kChat message ID 019ec243-6260-7264-aabb-3c25e27d3974 through the WebSocket receive path
• Staged diff check: git diff --cached --check
• Secret-pattern scan over staged diff produced no matches after replacing fixture token literals

Notes

• OpenClaw was temporarily pointed at this branch for testing and then restored to its exact published liquid-potassium@0.2.0 pin.