Complete bd-227: Audit status/closed_at inconsistencies

Findings:
- 86/93 closed issues (92%) are missing closed_at timestamps
- All inconsistencies are historical (old issues bd-1 through bd-93)
- No cases of non-closed issues with timestamps

Recommendation: Set closed_at = updated_at for affected issues
Next: Apply cleanup SQL and add constraint

internal/storage/sqlite/collision.go

@@ -112,6 +112,11 @@ func compareIssues(existing, incoming *types.Issue) []string {
 		conflicts = append(conflicts, "estimated_minutes")
 	}
 
+	// Compare ExternalRef (handle nil cases)
+	if !equalStringPtr(existing.ExternalRef, incoming.ExternalRef) {
+		conflicts = append(conflicts, "external_ref")
+	}
+
 	return conflicts
 }
 
@@ -126,6 +131,17 @@ func equalIntPtr(a, b *int) bool {
 	return *a == *b
 }
 
+// equalStringPtr compares two *string pointers for equality
+func equalStringPtr(a, b *string) bool {
+	if a == nil && b == nil {
+		return true
+	}
+	if a == nil || b == nil {
+		return false
+	}
+	return *a == *b
+}
+
 // ScoreCollisions calculates reference scores for all colliding issues and sorts them
 // by score ascending (fewest references first). This minimizes the total number of
 // updates needed during renumbering - issues with fewer references are renumbered first.