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/todo_simple/README.md

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+# todo_simple
+
+Tests one clear action item with responsible person and deadline.
+
+The difficult part is not adding extra scope: Nina explicitly says she will not touch the layout.
+
+Typical LLM mistakes:
+
+- Adding a layout update as a task.
+- Dropping the deadline.
+- Turning Omar's request into the task evidence instead of Nina's commitment.

tests/gold/todo_simple/expected.json

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+{
+  "facts": [
+    {
+      "fact": "The beta signup page points to the old privacy note.",
+      "evidence": "Omar: The beta signup page still points to the old privacy note."
+    }
+  ],
+  "decisions": [],
+  "todos": [
+    {
+      "task": "Update the privacy link on the beta signup page.",
+      "responsible": "Nina",
+      "deadline": "Thursday noon",
+      "evidence": "Nina: Yes, I will update the privacy link by Thursday noon."
+    }
+  ],
+  "questions": [],
+  "positions": [],
+  "technical": []
+}

tests/gold/todo_simple/transcript.txt

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+Omar: The beta signup page still points to the old privacy note.
+
+Nina: Yes, I saw that yesterday.
+
+Omar: Can you update the link before the partner demo?
+
+Nina: Yes, I will update the privacy link by Thursday noon.
+
+Omar: Great. Nothing else on that page from my side.
+
+Nina: I will only touch the link, not the layout.
+
+Omar: Fine.
+
+Nina: Then I have what I need.
+
+Omar: We can move on.
+
+Nina: Yes.