Establish prompt engineering baseline with Gold Standard tests

- introduce Gold Standard evaluation corpus
- document decision taxonomy
- define prompt-engineering methodology
- add regression workflow
- establish Prompt Version 2 baseline
- validate decision_simple, decision_deferred and decision_none

tests/gold/mixed_small/README.md

@@ -0,0 +1,11 @@
+# mixed_small
+
+Tests a compact realistic meeting containing all major extraction categories.
+
+The transcript includes facts, one explicit decision, one action item, one open question, one opinion, and a technical constraint.
+
+Typical LLM mistakes:
+
+- Applying the demo-only decision to production.
+- Treating Mateo's diagnosis as a fact instead of a position.
+- Creating a long-term normalizer decision even though it is explicitly open.