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

tools/regate.py

@@ -4,7 +4,7 @@ A margin argued from a distribution is a guess. This replays `outputs/run-202608
 — 32 real rollouts per environment, `brain-qwen38-dspark`, thinking off — through the
 shipped scorer, rebuilds each task from its seed, and asks the candidate predicate
 directly. The model's behaviour is held fixed and only the reward varies, which is a
-cleaner measurement than a fresh sample would be and does not touch spark-1.
+cleaner measurement than a fresh sample would be and does not touch the reference node.
 
 It reads traces and environment sources. It writes nothing.