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

MULTI_TURN.md

@@ -163,7 +163,7 @@ it is configured here.
 - `uv run eval @ configs/canary_trap.toml --dry-run` → exit 0, config written ✓
 - stable verifiers 0.3.1 with upstream harnesses and runtimes present ✓
 - Docker 29.1.3 running (tier 2/3 runtime) ✓
-- spark-1 `brain` on `http://100.127.247.67:8001/v1`, model id `brain` ✓
+- the reference node `brain` on `$SPARK_BASE_URL`, model id `brain` ✓
 - `prime` CLI installed (0.6.25; 0.6.26 available) ✓
 - `PRIME_API_KEY` ✗ — see above