feat: thread-bound subagents on Discord (#21805)

* docs: thread-bound subagents plan

* docs: add exact thread-bound subagent implementation touchpoints

* Docs: prioritize auto thread-bound subagent flow

* Docs: add ACP harness thread-binding extensions

* Discord: add thread-bound session routing and auto-bind spawn flow

* Subagents: add focus commands and ACP/session binding lifecycle hooks

* Tests: cover thread bindings, focus commands, and ACP unbind hooks

* Docs: add plugin-hook appendix for thread-bound subagents

* Plugins: add subagent lifecycle hook events

* Core: emit subagent lifecycle hooks and decouple Discord bindings

* Discord: handle subagent bind lifecycle via plugin hooks

* Subagents: unify completion finalizer and split registry modules

* Add subagent lifecycle events module

* Hooks: fix subagent ended context key

* Discord: share thread bindings across ESM and Jiti

* Subagents: add persistent sessions_spawn mode for thread-bound sessions

* Subagents: clarify thread intro and persistent completion copy

* test(subagents): stabilize sessions_spawn lifecycle cleanup assertions

* Discord: add thread-bound session TTL with auto-unfocus

* Subagents: fail session spawns when thread bind fails

* Subagents: cover thread session failure cleanup paths

* Session: add thread binding TTL config and /session ttl controls

* Tests: align discord reaction expectations

* Agent: persist sessionFile for keyed subagent sessions

* Discord: normalize imports after conflict resolution

* Sessions: centralize sessionFile resolve/persist helper

* Discord: harden thread-bound subagent session routing

* Rebase: resolve upstream/main conflicts

* Subagents: move thread binding into hooks and split bindings modules

* Docs: add channel-agnostic subagent routing hook plan

* Agents: decouple subagent routing from Discord

* Discord: refactor thread-bound subagent flows

* Subagents: prevent duplicate end hooks and orphaned failed sessions

* Refactor: split subagent command and provider phases

* Subagents: honor hook delivery target overrides

* Discord: add thread binding kill switches and refresh plan doc

* Discord: fix thread bind channel resolution

* Routing: centralize account id normalization

* Discord: clean up thread bindings on startup failures

* Discord: add startup cleanup regression tests

* Docs: add long-term thread-bound subagent architecture

* Docs: split session binding plan and dedupe thread-bound doc

* Subagents: add channel-agnostic session binding routing

* Subagents: stabilize announce completion routing tests

* Subagents: cover multi-bound completion routing

* Subagents: suppress lifecycle hooks on failed thread bind

* tests: fix discord provider mock typing regressions

* docs/protocol: sync slash command aliases and delete param models

* fix: add changelog entry for Discord thread-bound subagents (#21805) (thanks @onutc)

---------

Co-authored-by: Shadow <hi@shadowing.dev>

src/discord/send.webhook-activity.test.ts

@@ -0,0 +1,50 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { sendWebhookMessageDiscord } from "./send.js";
+
+const recordChannelActivityMock = vi.hoisted(() => vi.fn());
+
+vi.mock("../infra/channel-activity.js", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../infra/channel-activity.js")>();
+  return {
+    ...actual,
+    recordChannelActivity: (...args: unknown[]) => recordChannelActivityMock(...args),
+  };
+});
+
+describe("sendWebhookMessageDiscord activity", () => {
+  beforeEach(() => {
+    recordChannelActivityMock.mockReset();
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => {
+        return new Response(JSON.stringify({ id: "msg-1", channel_id: "thread-1" }), {
+          status: 200,
+          headers: { "content-type": "application/json" },
+        });
+      }),
+    );
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("records outbound channel activity for webhook sends", async () => {
+    const result = await sendWebhookMessageDiscord("hello world", {
+      webhookId: "wh-1",
+      webhookToken: "tok-1",
+      accountId: "runtime",
+      threadId: "thread-1",
+    });
+
+    expect(result).toEqual({
+      messageId: "msg-1",
+      channelId: "thread-1",
+    });
+    expect(recordChannelActivityMock).toHaveBeenCalledWith({
+      channel: "discord",
+      accountId: "runtime",
+      direction: "outbound",
+    });
+  });
+});