Correct Prometheus deployment docs for current metrics surface

[BennettSchwartz]

Summary

Deployment docs currently show Prometheus scraping localhost:9090 directly, but the daemon does not expose an HTTP /metrics endpoint.

Evidence

• docs/guide/deployment.md shows a Prometheus scrape target on localhost:9090.
• Codebase has no HTTP metrics handler (/metrics) or Prometheus exporter wiring.

Impact

Operators following docs can configure a non-functional scrape and assume metrics are unavailable or broken.

Proposed work

Either:

1. add an HTTP metrics endpoint/exporter, or
2. update docs to describe the currently supported metrics collection path (for example, polling GetMetrics via gRPC).

Acceptance criteria

• Deployment docs provide an implementation-accurate monitoring path.
• Prometheus section only documents supported integration patterns.