[BUG] Cowork Dispatch send_message no longer delivers to existing Cowork sessions (worked 461 times until Aug 10)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

This is about the **Cowork Dispatch agent in Claude Desktop**, not the Claude Code CLI.

Until 10 August, Dispatch could deliver messages into any of my existing local Cowork sessions via `send_message`. I used it daily to steer manually created per-project orchestrator chats from my phone. Now it refuses every target Dispatch did not spawn itself, while `list_sessions` and `read_transcript` still work on those same sessions - only writing is blocked.

From the Dispatch session own transcript:

- 468 `send_message` calls, **461 successful**, across **11 distinct sessions**
- only **12** `start_task`/`start_code_task` calls in the whole history, so most successful targets were never children
- `local_02ac95d7-2049-4082-a851-e70b0e10dd47`, created by hand in the UI, received **293** successful messages
- last success into a manual chat: **2026-08-10 17:30:52 UTC**; first refusal: **2026-08-21 10:08:50 UTC**

The error text changed too (below). The old wording pointed at `list_sessions`, i.e. any local session was a valid target.

### What Should Happen?

`send_message` should keep delivering to any local Cowork session that `list_sessions` reports, as it did until 10 August.

If restricting it to spawned children is intentional, it should be documented, and there should be a supported way to address an existing session. Feature request #41273 asks for exactly this and was closed as not planned - my logs show the capability already existed then.

### Error Messages/Logs

```shell
Before (up to 26 July 2026):
Session "..." not found. Use list_sessions to see available sessions.

Now (from 21 August 2026):
Session "..." not found. Valid session_ids are the ones returned by your start_task or start_code_task calls.
```

### Steps to Reproduce

1. In Claude Desktop, create a Cowork chat by hand in the UI (not through Dispatch).
2. In Dispatch, run `list_sessions` - the chat appears with `is_child: false`.
3. `read_transcript` on that session id works.
4. `send_message` to the same id fails: Session not found / valid ids are the ones returned by start_task.
5. A session created from the same Dispatch via `start_task` accepts `send_message` normally.

Reproduced on two Macs running 1.34493.1.

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

1.26832.0 (Claude Desktop)

### Claude Code Version

Claude Desktop 1.34493.1 (desktop app, not CLI)

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

Versions on this machine between last success and first refusal (from `[updater] Version changed since last launch`): 1.28929.0 (13 Aug), 1.30096.1 (14 Aug), 1.30096.5 (15 Aug), 1.32352.1 (18 Aug), 1.32885.1 (20 Aug), 1.34493.1 (21 Aug). I made no send attempts in that window, so I cannot narrow it further; release notes for 1.34493.0 and 1.34493.1 list no Cowork changes of this kind.

Support (Fin) called it an intentional restriction on Dispatch rather than a regression, while stating it had no information on whether it was a documented planned change. Note that the cross-session messaging docs it cited describe a different subsystem - ListAgents/SendMessage between CLI sessions over inbox sockets.

I have a CSV of all 468 calls (timestamps, target ids, titles, result text; no message content) available on request.

No repository file, test, or implementation entry point is identified, and the report explicitly distinguishes Claude Desktop from the Claude Code CLI. First confirm whether Cowork Dispatch is implemented in this repository; otherwise route the report to the desktop implementation. Reproduce the manual-session and spawned-session cases, with completion defined as send_message matching the documented intended behavior.

Written by the indexing model from the issue text.