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_DEFINITION.md

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+# Decision Definition
+
+A decision is any explicit agreement that creates a binding change in action,
+process, responsibility, approval status, timing, or next step.
+
+Included:
+
+- substantive decisions
+- organizational decisions
+- process decisions
+- approvals
+- rejections
+- deferrals
+- explicit agreement not to decide yet
+- explicit agreement to gather more information before deciding
+
+Excluded:
+
+- opinions
+- preferences
+- proposals without agreement
+- open questions
+- descriptions of the current state
+- explanations without commitment
+
+Important distinction:
+
+- "No decision was reached" means the meeting ended without an agreed outcome.
+- "The group decided to defer the decision" means the group explicitly agreed
+  on a process outcome: the substantive decision is postponed.
+
+These are not equivalent.