Add agent version to mcp node list

Thread the linker-injected version string into the Agent struct and
return it in the NodeStatus RPC. The CLI now dials each node and
displays the agent version alongside name and address.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

proto/mcp/v1/mcp.proto

@@ -257,6 +257,7 @@ message NodeStatusResponse {
   uint64 memory_free_bytes = 9;
   double cpu_usage_percent = 10;
   google.protobuf.Timestamp uptime_since = 11;
+  string agent_version = 12;
 }
 
 // --- Purge ---