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/agents/tools/subagents-tool.ts

@@ -7,6 +7,7 @@ import {
   sortSubagentRuns,
   type SubagentTargetResolution,
 } from "../../auto-reply/reply/subagents-utils.js";
+import { DEFAULT_SUBAGENT_MAX_SPAWN_DEPTH } from "../../config/agent-limits.js";
 import { loadConfig } from "../../config/config.js";
 import type { SessionEntry } from "../../config/sessions.js";
 import { loadSessionStore, resolveStorePath, updateSessionStore } from "../../config/sessions.js";
@@ -199,7 +200,8 @@ function resolveRequesterKey(params: {
   // Check if this sub-agent can spawn children (orchestrator).
   // If so, it should see its own children, not its parent's children.
   const callerDepth = getSubagentDepthFromSessionStore(callerSessionKey, { cfg: params.cfg });
-  const maxSpawnDepth = params.cfg.agents?.defaults?.subagents?.maxSpawnDepth ?? 1;
+  const maxSpawnDepth =
+    params.cfg.agents?.defaults?.subagents?.maxSpawnDepth ?? DEFAULT_SUBAGENT_MAX_SPAWN_DEPTH;
   if (callerDepth < maxSpawnDepth) {
     // Orchestrator sub-agent: use its own session key as requester
     // so it sees children it spawned.