The Responses API everywhere, driven against a real provider

The Chat Completions API is replaced by the Responses API everywhere in the harness, using as many of the protocol’s capabilities as possible, true to spec, including Coral’s cross-session prompt cache reuse. The real harness is test-driven against Coral with long conversations, multi-turn work and real tasks on real codebases; the output is evaluated and improved; every discrepancy in Coral’s implementation is recorded (in its provider profile and its generated report, UR-13). Caching never costs a harness function (UR-03).