Migrate plugin channels to protomux-rpc only

Replace dual message-channel + RPC setup with a single registerPluginProtocol
path, unified sdk.channels (register/request/event/broadcast), RPC keepalive
(__p2ns.ping/pong), and core request lifecycle on RPC open. Update
global.profile and example.plugin, admin Plugin RPC stats, docs, and
test:plugin-rpc. Breaking: upgrade all peers together; no legacy adapters.

proxy-server/p2ns_proxy_server.js

@@ -827,8 +827,8 @@ async function setupP2NS() {
     onConsensusRemoveConflictClaim: proxySwarmHandlers.onConsensusRemoveConflictClaim
   });
 
-  channelManager.registerPluginChannel(coreRpc.CORE_DOMAIN, 'request', {
-    encoding: 'string',
+  channelManager.registerPluginProtocol(coreRpc.CORE_DOMAIN, 'request', {
+    methods: {},
     autoReconnect: true
   });