Add rate limiting with Bottleneck

[burner-agent]

Summary

Integrate SGrondin/bottleneck (2k stars) for robust rate limiting across Discord API calls, Claude CLI invocations, and webhook dispatches.

Motivation

During busy RALPH loops, Burner can hit Discord rate limits. The current src/queue.ts handles basic queuing, but Bottleneck provides battle-tested rate limiting with priorities, clustering support, and reservoir-based throttling that would prevent API abuse and enable better resource allocation.

Proposal

• Add bottleneck as a dependency
• Wrap Discord API calls with a Bottleneck limiter matching Discord's rate limits
• Wrap Claude CLI spawns with a concurrency limiter to prevent resource exhaustion
• Add priority levels so operator commands bypass queue during high load

References

• https://github.com/SGrondin/bottleneck