Surface rate-limit telemetry from `discover`

[JSONbored]

Problem: The discovery fanout already computes GitHub rate-limit remaining/reset internally, but discover's result object never surfaces it — an operator has no visibility into how close they are to being rate-limited.

Area: AMS / Observability

Proposal: Include the already-computed rate-limit telemetry in discover's output.

Deliverables:

• Rate-limit remaining/reset visible in both human-readable and --json output of discover.

Acceptance criteria:

• Running discover shows the current rate-limit state without needing a separate command.

[linear-code]

JSO-3497