feat: init inbound webhook for Rocket.Chat channel plugin

[dodaa08]

What this does

Adds a unified Rocket.Chat plugin to OpenClaw that handles both sending and receiving messages without needing an external bridge server.

How the webhook works

RC client → POST /rocketchat/webhook
↓ validates x-rocketchat-livechat-token
↓ filters out bot/self messages
↓ api.gateway.dispatchInbound(...)
↓ OpenClaw routes message inbound

Related to #2

Still figuring out

Trying to get registerChannel to correctly wire up with the custom webhook so RC client can call it end to end. The outbound via REST API works, inbound webhook is set up just need to confirm api.gateway.dispatchInbound is being picked up correctly by OpenClaw once the channel is registered.

Env vars needed

Var	Purpose
RC_URL	RC server URL
RC_AUTH_TOKEN	Bot auth token
RC_USER_ID	Bot user ID
DEFAULT_ROOM	Fallback room ID
RC_WEBHOOK_SECRET	Webhook validation token