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.

tests/gold/decision_simple/expected.json

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+{
+  "facts": [],
+  "decisions": [
+    {
+      "decision": "The welcome email will be sent after account activation.",
+      "evidence": "So are we agreed that the welcome email moves to after activation? Ben: Agreed. Cara: Yes, let's do that."
+    }
+  ],
+  "todos": [],
+  "questions": [],
+  "positions": [],
+  "technical": []
+}

tests/gold/decision_simple/transcript.txt

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+Anna: Before we leave the onboarding flow, can we settle the email step?
+
+Ben: I still think the welcome email should go out after account activation, not before.
+
+Cara: Yes, before activation it keeps confusing people.
+
+Anna: So are we agreed that the welcome email moves to after activation?
+
+Ben: Agreed.
+
+Cara: Yes, let's do that.
+
+Anna: Good. Then that is the decision for this release.
+
+Ben: Separate note on the copy: I am not proposing any wording decision today.
+
+Cara: Same here, no wording proposal from me.
+
+Anna: Okay, then the only decision is the timing after activation.