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/question_simple/transcript.txt

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+Kai: The vendor invoice came in this morning.
+
+Ruth: Does it include the extra support hours from March?
+
+Kai: I don't know. The line item just says support package.
+
+Ruth: Then that is still open.
+
+Kai: I can ask finance, but I am not taking that as a task yet.
+
+Ruth: Fine. Let's leave it as an open question for now.
+
+Kai: Understood.
+
+Ruth: Anything else on invoices?
+
+Kai: No.
+
+Ruth: Then next item.