docs: redact the reference node's address and hostname

The eval docs named spark-1 by fleet hostname and published its inference
endpoint (a tailnet address and port) plus the fact that it answers without
an API key. This repo is PUBLIC, so that was a standing disclosure of an
internal, unauthenticated service to anonymous readers.

Endpoints now come from $SPARK_BASE_URL, which the operator exports, and the
host is referred to as "the reference node" -- the convention the compute
repo's docs already use.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012AaUFYkUTsJn1fnJ89qbvW

docs/PRIME_FIRST.md

@@ -5,7 +5,7 @@ harness, trainer, runtime marketplace, or environment hub.
 
 - Every environment pins stable `verifiers==0.3.1`.
 - Pure-trace environments select Verifiers' upstream `null` harness.
-- Local model-free and spark-1 runs name the `subprocess` runtime explicitly so
+- Local model-free and the reference node runs name the `subprocess` runtime explicitly so
   tests never provision a paid Prime sandbox.
 - Prime Hub is the public distribution target for approved environment wheels.
 - Prime-RL consumes the same taskset packages for training; Forge owns the