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_family_models.py

@@ -67,3 +67,47 @@ def test_a_canon_ideas_scope_lives_on_the_idea_and_nowhere_else():
 
     assert "topic_id" in family.FamilyIdea.__table__.c
     assert not {c.name for c in RulebookTopic.__table__.c} & {"platform_id", "platform_ids"}
+
+
+# --- a decision read as precedent (#5131) ------------------------------------------
+
+def _assessment():
+    reason = "the APK is built in CI and signed with the one release key"
+    return family.FamilyDecision(
+        id=7, idea_id=3, project_id=5, action="assess", reason=reason,
+        before={"status": "unassessed", "reason": "", "canon_version": None},
+        after={"status": "adopted", "reason": reason, "canon_version": 2},
+        evidence={"evidence": ["ci.yml::android-release"], "source": "shapes"},
+        precedent_ids=[4, 6], decided_via="agent", user_id=1,
+    )
+
+
+def test_a_brief_carries_the_reason_once():
+    brief = _assessment().to_brief()
+    assert brief["reason"].startswith("the APK is built")
+    assert brief["before"] == {"status": "unassessed", "canon_version": None}
+    assert brief["after"] == {"status": "adopted", "canon_version": 2}
+    # Nothing in it says the reason a second time.
+    assert repr(brief).count("the APK is built") == 1
+
+
+def test_a_brief_keeps_the_cited_evidence_and_drops_the_rest():
+    promotion = family.FamilyDecision(
+        id=8, idea_id=3, project_id=None, action="promote", reason="proven twice",
+        before=None,
+        after={"status": "canon", "applies_when": "an app that ships its own APK",
+               "canon_version": 1, "platforms": ["android-app"]},
+        evidence={"criteria": {"proven": "a long paragraph"}, "evidence": ["#12"],
+                  "ledger_rows_opened": 3},
+    )
+    brief = promotion.to_brief()
+    assert brief["evidence"] == ["#12"]
+    assert brief["before"] is None
+    assert brief["after"] == {"status": "canon", "canon_version": 1,
+                              "platforms": ["android-app"]}
+
+
+def test_the_full_row_is_still_the_audit_shape():
+    full = _assessment().to_dict()
+    assert full["after"]["reason"] == full["reason"]
+    assert full["evidence"]["source"] == "shapes" and full["precedent_ids"] == [4, 6]