refactor: centralize gateway module and address review fixes

Move the gateway runtime files into a dedicated core/gateway module and fix follow-up issues in session deletion, history import batching, message IDs, and legacy thread parsing.

Fixes #253

Co-authored-by: Codex <noreply@openai.com>

packages/coding-agent/src/core/gateway/index.ts

@@ -0,0 +1,19 @@
+export {
+  createGatewaySessionManager,
+  GatewayRuntime,
+  getActiveGatewayRuntime,
+  sanitizeSessionKey,
+  setActiveGatewayRuntime,
+} from "./runtime.js";
+export type {
+  ChannelStatus,
+  GatewayConfig,
+  GatewayMessageRequest,
+  GatewayMessageResult,
+  GatewayRuntimeOptions,
+  GatewaySessionFactory,
+  GatewaySessionSnapshot,
+  HistoryMessage,
+  HistoryPart,
+  ModelInfo,
+} from "./runtime.js";