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/transcript.txt

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