Steering the main chat irreversibly kills all running background subagents

### What happened

I sent a steering message to the main chat while two background subagents were running. Sending that message terminated **both** subagents — including the one entirely unrelated to what I was steering.

Afterwards they could not be resumed. `SendMessage` to the agent returned:

```
Agent was stopped by the user and won't be resumed.
Treat its work as cancelled; only launch a new agent if the user explicitly asks.
```

So several minutes of subagent work were lost, and the agent had to be relaunched from scratch with the same prompt.

### Expected behaviour

1. Steering / sending a message to the main chat should not terminate background subagents at all — they are independent units of work, and interacting with the main thread is not a signal to abandon them.
2. If a subagent *is* stopped, it should be resumable. The `SendMessage` tool description explicitly promises this ("a send resumes it from its transcript"), and the transcript still exists on disk — but the runtime marks the agent as permanently cancelled, so resuming is impossible.

At minimum these two should agree: either the documented resume-by-message behaviour works, or the tool description should not promise it.

### Impact

Any interaction with the main chat becomes risky while background work is in flight. In a long-running session this silently discards work and forces duplicate token spend on a relaunch. It also makes the documented resume-by-message behaviour unreliable, so there is no way to recover an accidentally stopped agent.

### Environment

- Claude Code, desktop app
- Model: Opus 5
- Subagents launched via the `Agent` tool with `run_in_background` (default)

Start by reproducing the interaction between the Agent tool with run_in_background, main-chat steering, and SendMessage; inspect the agent transcript and runtime state after a stop. Done means steering the main chat leaves unrelated background subagents running, and a stopped subagent can be resumed from its transcript as promised.

Written by the indexing model from the issue text.