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/decision_simple/README.md

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+# decision_simple
+
+Tests one explicit decision with clear agreement language.
+
+The difficult part is separating the decision from nearby rationale about user confusion and from the non-decision statement that the copy can stay unchanged for now.
+
+Typical LLM mistakes:
+
+- Extracting the rationale as a separate decision.
+- Treating "copy can stay as it is" as a formal decision.
+- Losing the evidence that shows explicit agreement.