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/PROMPT_ENGINEERING_METHODOLOGY.md

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+# Prompt Engineering Methodology
+
+Prompt engineering uses the Gold Standard corpus as the reference. The corpus is
+not adjusted to make a prompt pass.
+
+Rules:
+
+1. Only one prompt change is allowed per iteration.
+2. Only one gold test case may be optimized at a time.
+3. Every prompt modification must be validated immediately.
+4. A prompt modification is acceptable only if it improves the current target
+   and does not degrade any previously passing gold test.
+5. Never modify `expected.json` to make a prompt pass.
+6. Prompt engineering edits prompt files only. Python code changes require a
+   separate explicit task.
+7. Maintain a prompt evolution log for every iteration.
+8. If a prompt cannot improve a test after several small iterations, stop and
+   analyze the root cause.
+9. Prompt changes must be generally applicable and must not special-case one
+   transcript.
+10. If two consecutive prompt iterations fail to improve the current target,
+    stop further prompt modifications and classify the root cause.
+11. If a prompt produces unexpected behavior, first verify whether the targeted
+    gold test has an objectively unique ground truth.
+
+Prompt Version 2 baseline:
+
+- `decision_simple`: passing
+- `decision_deferred`: passing
+- `decision_none`: passing
+
+Prompt Version 2 adds explicit support for process decisions where the group
+agrees to defer a substantive decision until additional information is
+available.