add reproducible cross-model benchmark matrix

docs/MODEL_MATRIX.md

@@ -0,0 +1,60 @@
+# Model and Harness Comparisons
+
+T3MP3ST model comparisons are built from committed benchmark receipts. The
+matrix command aggregates existing artifacts; it never starts containers,
+calls a model, or contacts a target.
+
+## Reproduce the committed comparison
+
+```bash
+npm run bench:model-matrix
+npm run verify:model-matrix
+```
+
+The committed Cybench matrix compares Claude Opus 4.7 and 4.8 on the same 15
+task IDs, `live-tools` harness label, artifact schema, and pass@1 policy. Its
+JSON contains every source artifact path; its Markdown output provides the
+human-readable table.
+
+Outcome categories stay separate:
+
+- `success`: the committed strict oracle verdict detected the expected flag;
+- `failure`: the benchmark ran but did not satisfy the oracle;
+- `abstention`: the artifact explicitly records a refusal, abstention, or
+  no-action outcome;
+- `infrastructure_error`: the run failed because of a timeout, API, spawn, or
+  equivalent harness error;
+- `skipped`: the expected artifact is unavailable.
+
+These are system results: model, provider, harness, runtime, tools, target
+class, and attempt policy all matter. Historical dates and model versions also
+differ. Do not interpret the table as an isolated model-quality ranking.
+
+## Run arbitrary model variants
+
+The refusal-frontier harness accepts any comma-separated OpenRouter model IDs
+without source edits:
+
+```bash
+# Offline: prints the exact shared corpus and call count; spends no API quota.
+node scripts/refusal-frontier.mjs \
+  --models anthropic/model-a,openai/model-b \
+  --classes crypto,memory \
+  --n 3 \
+  --dry-run
+
+# API-backed and cost-bearing: requires OPENROUTER_API_KEY.
+node scripts/refusal-frontier.mjs \
+  --models anthropic/model-a,openai/model-b \
+  --classes crypto,memory \
+  --n 3 \
+  --at comparison-2026-07
+```
+
+Every selected model receives the same classes, five-rung corpus, sample count,
+judge, and concurrency settings. JSON output retains raw responses and judge
+rationales; Markdown output renders model-by-model and cross-model deltas.
+
+Use the fields in [CONTRIBUTION_RECEIPTS](CONTRIBUTION_RECEIPTS.md) when adding
+new results. Never commit credentials or private target data, and never present
+live external runs without a target-specific authorization receipt.