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_none/README.md

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+# decision_none
+
+Tests a true decision-negative meeting segment.
+
+The transcript contains discussion, competing preferences, and possible options
+for the weekly dashboard. No participant approves an option, rejects an option
+on behalf of the group, assigns a follow-up, agrees to gather more information,
+or explicitly decides to defer the decision.
+
+Ground truth:
+
+- No decision was reached.
+- No process decision was reached.
+- No agreed next step was created.
+
+Typical LLM mistakes:
+
+- Treating a preference as a decision.
+- Treating a proposed option as the selected option.
+- Treating the topic change as an implicit deferral decision.
+- Creating an action item for Dana even though she is only mentioned as absent.