Rewrites the utility-process worker to drive pi-agent-core's Agent class
against pi-ai models and pi-coding-agent tools. The entry file keeps the
queue and cancellation state, delegates runs to WorkerRunner, and routes
RPC responses through WorkerRpcClient so beforeToolCall/afterToolCall
approvals and prepareRun credential resolution travel back to main's
RuntimeBridge. Worker message schema gains `agent-event` + `rpc-request`
envelopes, and run-finished no longer depends on the deleted sessionCodec.
