Wire the existing miner-prediction Prometheus renderer into a real command

[JSONbored]

Problem: A fully-built Prometheus text-exposition renderer for miner prediction accuracy already exists, explicitly designed for cron/scrape use — but nothing in the CLI ever calls it.

Area: AMS / Observability

Proposal: Add a CLI subcommand (e.g. metrics) that calls the existing renderer and prints Prometheus-format output to stdout, suitable for a scrape wrapper or cron redirect.

Deliverables:

• A working metrics subcommand backed by the existing renderer.

Acceptance criteria:

• Running the new subcommand produces valid Prometheus text-exposition output.

Boundaries:

• Don't change the renderer itself — it's already correct, just uncalled.

[linear-code]

JSO-3498