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/technical_simple/transcript.txt

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+Sofia: The mobile build failed again on the staging runner.
+
+Nils: Same error as yesterday?
+
+Sofia: No, different. The iOS job now fails during code signing.
+
+Nils: The certificate itself is valid until October.
+
+Sofia: Right. The problem is that the runner uses the old keychain path.
+
+Nils: So it is a runner configuration issue, not a certificate expiry issue.
+
+Sofia: Exactly.
+
+Nils: We are not deciding the fix today.
+
+Sofia: Okay.
+
+Nils: Next item.