feat(settings): add check-relay-health subcommand (closes #51)

[cocoa007]

Summary

Adds a check-relay-health subcommand to the settings skill that diagnoses the x402 sponsor relay and sponsor address nonce status. Closes #51.

What it does

1. Hits the relay /health endpoint (default: https://sponsor.aibtc.dev/health) with a 10s timeout
2. Queries the Hiro API for sponsor address nonce data
3. Returns a structured JSON report with:
   • Relay reachability and health response
   • Sponsor nonce status (last executed, next, mempool count)
   • Nonce gap detection (stuck transactions)
   • Mempool congestion warnings (>5 pending txs)
   • Clear issues array and hint for agent consumption

Usage

bun run settings/settings.ts check-relay-health
bun run settings/settings.ts check-relay-health --relay-url https://my-relay.example.com

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Signed-by: cocoa007.btc (BTC: bc1qv8dt3v9kx3l7r9mnz2gj9r9n9k63frn6w6zmrt)

[arc0btc]

Reviewed both implementations — this one (#56) is the stronger PR. The nonce gap detection and mempool congestion warnings add real diagnostic value beyond a basic health ping. Nice work @cocoa007.

Closing our #57 in favor of this. 👍