refactor(worker): use pi-mono Agent directly

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.

src/main/worker/worker-message-handler.ts

@@ -1,3 +1,4 @@
+import type { RuntimeBridge } from "../agent-runtime";
 import type { workerMessageSchema } from "./worker-types";
 import { handleRunLog } from "./handlers/run-log-handler";
 import { handleRunStatus } from "./handlers/run-status-handler";
@@ -6,8 +7,8 @@ import type { HandlerContext, HandlerDeps } from "./handlers/handler-context";
 
 export async function handleWorkerMessage(
   message: unknown,
-  _worker: ReturnType<typeof import("electron").utilityProcess.fork> | null,
-  deps: HandlerDeps,
+  worker: ReturnType<typeof import("electron").utilityProcess.fork> | null,
+  deps: HandlerDeps & { getRuntimeBridge?: () => RuntimeBridge | null },
   workerMessageSchemaRef: typeof workerMessageSchema,
   destroyed: boolean,
   MAX_RETRY_ATTEMPTS: number,
@@ -44,5 +45,38 @@ export async function handleWorkerMessage(
     case "run-finished":
       handleRunFinished(event, deps, context);
       return;
+    case "agent-event": {
+      const bridge = deps.getRuntimeBridge?.();
+      if (bridge) {
+        bridge.handleAgentEvent({
+          runId: event.runId,
+          agentId: event.agentId,
+          companyId: event.companyId,
+          taskId: event.taskId,
+          emittedAt: event.emittedAt,
+          event: event.event,
+        });
+      }
+      return;
+    }
+    case "rpc-request": {
+      const bridge = deps.getRuntimeBridge?.();
+      const response = bridge
+        ? await bridge.handleRpc({
+            rpcId: event.rpcId,
+            runId: event.runId,
+            method: event.method,
+            payload: event.payload,
+          })
+        : { ok: false as const, error: "RuntimeBridge not initialised." };
+      try {
+        worker?.postMessage({ type: "rpc-response", rpcId: event.rpcId, response });
+      } catch (error) {
+        deps.logger.error(
+          `[worker] rpc-response postMessage failed: ${error instanceof Error ? error.message : String(error)}`,
+        );
+      }
+      return;
+    }
   }
 }