docs(rfc): Fold the queue pause decision into the RFC

[manjari25]

Why?

The pause RFC still presented two options as an open comparison, but the stack in #862 to #865 and the e2e tests implement one of them. An RFC describes what is on main, so it now records the design that shipped and only the alternative it rejected.

What?

• Rewrites the pause RFC around the shipped design: a separate queuepause extension, reject (FailedPrecondition, carrying the operator's reason) and freeze, both failing closed on a read error. A cancelled context is passed through.
• Documents the noop, file and fake stores, QUEUE_PAUSE_CONFIG_PATH, and the shared wiring helper.
• Adds the requirements an implementation of queuepause.Store must meet, such as failing closed on read errors, strict parsing, and failing at construction on bad configuration.
• States that the gateway's own request-log consumer is not frozen, so status and history keep materializing during a freeze.
• Keeps the queue-config option only as a short rejected alternative, and drops the resolved fail-open question.
• Corrects the consumer gate RFC: orchestrator stages share one consumer group, and production pause is a separate mechanism that wraps the gate. Updates the RFC index entry.

Test Plan

Docs only, no runnable change. make lint passes.

Issue