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

PLAN.md

@@ -54,7 +54,7 @@ identity across all six projects; no per-project byline.
 the Prime Intellect Hub yet. Workstream E is parked — not cancelled, not blocked on
 anything technical. `PRIME_API_KEY` is therefore not urgent.
 
-**✅ 7 — Production brain: Qwen3.8-27B.** Settled 2026-08-21. spark-1 serves
+**✅ 7 — Production brain: Qwen3.8-27B.** Settled 2026-08-21. the reference node serves
 `brain-qwen38-dspark`; that is the default. The Nemotron baseline is ARCHIVED, not
 deleted, and Arena's seven scores get re-run and republished against Qwen.
 ⚠️ **`--model brain` does NOT 404 — it silently succeeds and records `brain` in the
@@ -118,7 +118,7 @@ trap) already exists, and it is the smallest change that yields a genuinely new
 measurement.
 
 Deliverable: a `vf.Env` subclass exported from the wheel's `__init__.py`, a config,
-and a measured run against spark-1's `brain`. Pattern to copy:
+and a measured run against the reference node's `brain`. Pattern to copy:
 `verifiers/v1/tasksets/textarena/taskset.py`.
 
 This workstream is where **rule 4 — the budget must bind** gets written for real.
@@ -218,7 +218,7 @@ files. Everything downstream of B fans out four ways.
 - `uv run eval @ configs/<env>.toml --dry-run` → exit 0 ✓
 - stable verifiers 0.3.1, using its upstream `null` harness and runtimes ✓
 - Docker 29.1.3 (tier 2/3 only; tier 1 needs none) ✓
-- spark-1 `brain` at `http://100.127.247.67:8001/v1`, model id `brain` ✓
+- the reference node `brain` at `$SPARK_BASE_URL`, model id `brain` ✓
 - `prime` CLI 0.6.25 ✓
 - `PRIME_API_KEY` ✗ — blocks E, which is parked anyway