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/mixed_small/README.md

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+# mixed_small
+
+Tests a compact realistic meeting containing all major extraction categories.
+
+The transcript includes facts, one explicit decision, one action item, one open question, one opinion, and a technical constraint.
+
+Typical LLM mistakes:
+
+- Applying the demo-only decision to production.
+- Treating Mateo's diagnosis as a fact instead of a position.
+- Creating a long-term normalizer decision even though it is explicitly open.

tests/gold/mixed_small/expected.json

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+{
+  "facts": [
+    {
+      "fact": "The staging import handled 12,000 rows last night.",
+      "evidence": "Priya: The staging import handled 12,000 rows last night."
+    },
+    {
+      "fact": "The staging import took 48 minutes.",
+      "evidence": "Mateo: It finished, but it took 48 minutes."
+    },
+    {
+      "fact": "The partner demo target is 30 minutes.",
+      "evidence": "Lena: Yes, that is still the demo target."
+    }
+  ],
+  "decisions": [
+    {
+      "decision": "Address normalization will be disabled for the demo import only.",
+      "evidence": "Can we agree to disable address normalization for the demo import only? Lena: Yes, for the demo import only. Mateo: Agreed."
+    }
+  ],
+  "todos": [
+    {
+      "task": "Update the demo import configuration.",
+      "responsible": "Mateo",
+      "deadline": "Friday noon",
+      "evidence": "Mateo: I will do that before Friday noon."
+    }
+  ],
+  "questions": [
+    {
+      "question": "Whether a faster normalizer is needed after the demo.",
+      "evidence": "Lena: And the open question is whether we need a faster normalizer after the demo."
+    }
+  ],
+  "positions": [
+    {
+      "speaker": "Mateo",
+      "position": "Mateo thinks address normalization is the slow part.",
+      "evidence": "Mateo: I think the slow part is address normalization."
+    }
+  ],
+  "technical": [
+    {
+      "subject": "Demo import configuration",
+      "statement": "Address normalization is disabled only for the demo import; production imports keep full normalization.",
+      "evidence": "for the demo import only. Production imports keep the full normalization."
+    }
+  ]
+}

tests/gold/mixed_small/transcript.txt

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+Priya: The staging import handled 12,000 rows last night.
+
+Mateo: It finished, but it took 48 minutes.
+
+Priya: The limit for the partner demo is 30 minutes, right?
+
+Lena: Yes, that is still the demo target.
+
+Mateo: I think the slow part is address normalization.
+
+Priya: Can we agree to disable address normalization for the demo import only?
+
+Lena: Yes, for the demo import only.
+
+Mateo: Agreed. Production imports keep the full normalization.
+
+Priya: Mateo, please update the demo config before Friday noon.
+
+Mateo: I will do that before Friday noon.
+
+Lena: And the open question is whether we need a faster normalizer after the demo.
+
+Priya: Capture that, but no decision on the long-term fix today.