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/expected.json

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+{
+  "facts": [
+    {
+      "fact": "The vendor invoice came in this morning.",
+      "evidence": "Kai: The vendor invoice came in this morning."
+    },
+    {
+      "fact": "The invoice line item says support package.",
+      "evidence": "Kai: I don't know. The line item just says support package."
+    }
+  ],
+  "decisions": [
+    {
+      "decision": "The support-hours invoice issue will remain an open question for now.",
+      "evidence": "Ruth: Fine. Let's leave it as an open question for now."
+    }
+  ],
+  "todos": [],
+  "questions": [
+    {
+      "question": "Does the vendor invoice include the extra support hours from March?",
+      "evidence": "Ruth: Does it include the extra support hours from March?"
+    }
+  ],
+  "positions": [],
+  "technical": []
+}