fix(family): undoing the decision that created an idea leaves no applicability or scope behind (#4989)

When the undone decision had no recorded before-state, the synthesized prior copied the canon idea's current applies_when and platforms into a retired row. Before that decision the record was not an idea, so the restored state now has neither. Caught by test_undoing_a_promotion_restores_the_prior_state_and_keeps_judged_rows (run 8281).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/services/family.py

@@ -620,9 +620,12 @@ async def undo(user_id: int, decision_id: int, *, reason: str, decided_via: str
             raise ValueError(f"decision {decision_id} cannot be undone: {why}")
         idea = await session.get(FamilyIdea, target.idea_id)
         before = await _snapshot(session, idea)
+        # No recorded prior state means the decision CREATED the idea: before
+        # it, the record had no applicability test and no scope. It comes back
+        # as retired, with neither, rather than being deleted with its log.
         prior = target.before or {
-            "status": "retired", "applies_when": idea.applies_when or "",
-            "canon_version": idea.canon_version, "platforms": before["platforms"],
+            "status": "retired", "applies_when": "",
+            "canon_version": idea.canon_version, "platforms": [],
         }
         if prior["status"] == "canon" and not (prior.get("applies_when") or "").strip():
             raise ValueError("the recorded prior state is canon with no 'applies when'")