Add M-D2-CST token-cost metric with committed model price sheet

benchmarks/baselines/model-prices.toml

@@ -0,0 +1,24 @@
+# USD per million tokens; source: Anthropic pricing, recorded 2026-07-31
+# Governed by specs/MetricsAndScenarios.md §1a — refresh on price change or
+# after 90 days, or new M-D2-CST 'better' verdicts are invalid.
+
+[claude-fable-5]
+input = 10.00
+output = 50.00
+
+[claude-opus-5]
+input = 5.00
+output = 25.00
+
+[claude-sonnet-5]
+input = 3.00    # intro 2.00 through 2026-08-31
+output = 15.00  # intro 10.00 through 2026-08-31
+
+[claude-haiku-4-5]
+input = 1.00
+output = 5.00
+
+[cache]                 # multipliers on the input price
+read = 0.1
+write_5m = 1.25
+write_1h = 2.0