Reorganize worker runtime layer into worker/runtime/ (M8 part 2/3)

Groups the pi Agent lifecycle, factory, signature contract, and env/redaction helpers into src/main/worker/runtime/. WorkerRunner is the module's public entry and lives in runtime/index.ts; pi-agent-factory and pi-hook-contract stay named for their role as visible runtime boundaries; helpers.ts carries ParentMessage/ChildMessage/RunJob types plus redactor + env override utilities. entry/index.ts and agent-runtime/runtime-message-injector.ts updated to import from the new paths.

src/main/agent-runtime/runtime-message-injector.ts

@@ -26,7 +26,7 @@
 import { randomUUID } from "node:crypto";
 import type { AppDatabase } from "@main/database";
 import type { AgentMessageRecord } from "@shared/types";
-import type { ParentMessage } from "@main/worker/worker-helpers";
+import type { ParentMessage } from "@main/worker/runtime/helpers";
 import type { RuntimeBridge } from "./runtime-bridge";
 
 export interface RuntimeMessageInjectorDeps {