Remove sendVatCommand and enqueue vat calls in Control Panel

[sirtimid]

The Kernel.sendVatCommand method currently bypasses the crank and sends commands directly to vats. This was introduced during early development as scaffolding, but it breaks the crank-based delivery model and undermines consistency in kernel-vat interactions.

We should:

• Remove the sendVatCommand RPC handler and kernel method
• Update the control-panel UI to call queueMessage with target, method, and params
• Adjust tests and examples to expect the new RPC

All vat messaging will now flow through the crank queue for consistent ordering and GC semantics.

[rekmarks]

Re: ping, see #490.