Add traceable case interpretation with validated LLM output

docs/llm-gateway-v1.md

@@ -2,8 +2,10 @@
 
 This milestone implements architecture section 13: configuration, provider transport,
 ordered failover, bounded retries, schema validation, metadata and lifecycle checkpoints.
-It does not implement Case Interpretation, hypotheses or Investigation Tools. The
-existing CaseBuilder remains deterministic and does not start an LLM request on its own.
+The gateway itself does not interpret cases, generate hypotheses or execute Investigation
+Tools. The separate CaseInterpreter now supplies the first domain prompt and schema
+(see case-interpretation-v1.md). The existing CaseBuilder remains deterministic and does
+not start an LLM request on its own.
 
 ## Configuration
 
@@ -135,5 +137,5 @@ result = await runner.run(case_id, package_id, operation_id="transport-test-v1",
 ```
 
 Tests use deterministic fake adapters and an actual local HTTP server. No cloud
-credentials, paid calls or real models are required. The next domain milestone is
-CaseInterpreter, whose versioned prompt and output schema will use this gateway.
+credentials, paid calls or real models are required. CaseInterpreter uses this gateway
+with its own versioned prompt and output schema.