refactor(agents): bind subagent threads in core (#88416)

Move subagent thread binding ownership into core so session-mode spawns prepare channel bindings before launching the child agent. Deprecate the legacy subagent_spawning SDK hook in code, compatibility metadata, diagnostics, and plugin docs; plugin authors should observe subagent_spawned instead.

Verification:
- node scripts/run-vitest.mjs src/agents/sessions-spawn-hooks.test.ts src/agents/subagent-spawn.thread-binding.test.ts src/agents/subagent-spawn.workspace.test.ts src/agents/subagent-spawn.mode-session-diagnostics.test.ts
- node scripts/run-tsgo.mjs -p tsconfig.core.json --incremental --tsBuildInfoFile .artifacts/tsgo-cache/core.tsbuildinfo
- git diff --check
- .agents/skills/autoreview/scripts/autoreview --mode local
- CI run 26693808952 green, including checks-node-agentic-agents-core and checks-node-agentic-plugin-sdk

extensions/feishu/src/subagent-hooks.test.ts

@@ -5,6 +5,7 @@ import {
 import { beforeEach, describe, expect, it } from "vitest";
 import type { ClawdbotConfig, OpenClawPluginApi } from "../runtime-api.js";
 import { registerFeishuSubagentHooks } from "../subagent-hooks-api.js";
+import { handleFeishuSubagentSpawning } from "./subagent-hooks.js";
 import {
   createFeishuThreadBindingManager,
   testing as threadBindingTesting,
@@ -18,7 +19,10 @@ const baseConfig: ClawdbotConfig = {
 function registerHandlersForTest(config: Record<string, unknown> = baseConfig) {
   return registerHookHandlersForTest<OpenClawPluginApi>({
     config,
-    register: registerFeishuSubagentHooks,
+    register: (api) => {
+      registerFeishuSubagentHooks(api);
+      api.on("subagent_spawning", (event, ctx) => handleFeishuSubagentSpawning(event, ctx));
+    },
   });
 }