[Feature Request] Add rate limiting and cost controls for concurrent subagent spawning

**Bug Description**
Burnt all my limit on purpose by spawning a large number of subagents

**Environment Info**
- Platform: darwin
- Terminal: WarpTerminal
- Version: 2.1.270
- Feedback ID: 244a0d8c-0564-4757-8ecd-e9d7d90dcd1e

**Errors**
```json
[]
```

The issue names no files, tests, or entry points and provides no acceptance criteria. Start by locating concurrent subagent spawning and usage-limit handling, then define the rate and cost-control behavior and tests needed to show that large spawns cannot unexpectedly exhaust the user's limit.

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