fix(family): a decision read as precedent says its reason once - to_brief for precedents and idea history; the decision log keeps the full row (#5131)

Each decision stored its reason three times (reason, before.reason, after.reason)
and a promotion carried the whole criteria reasoning in evidence. Precedents
(assess, promote, get_family_adoption, get_family_idea) and get_family_idea's
history now serve FamilyDecision.to_brief(): reason once, before/after as status,
canon_version and platforms, evidence as the cited list. list_family_decisions
and the decision echoed back by a write keep to_dict. The precedent count was
already bounded at five.

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

tests/test_integration_family_adoption.py

@@ -236,6 +236,9 @@ async def test_another_projects_answer_is_recorded_as_precedent(family):
     assert first["decision"]["id"] in second["decision"]["precedent_ids"]
     assert second["precedents"][0]["relation"] == "same idea, another project"
     assert second["precedents"][0]["project_title"] == "android one"
+    # A precedent says its reason once: before/after are state only (#5131).
+    assert second["precedents"][0]["reason"] == "the source app"
+    assert "reason" not in second["precedents"][0]["after"]
 
 
 # --- recheck --------------------------------------------------------------------------