address self-review findings on PR #89

MINOR fixes:
- docs/DESIGN-57-yaml-persona.md: fix Error Cases table entry to reflect
  custom AST walk (checkYAMLDepth) instead of stale library-level reference
- review/persona.go: add EOF check after JSON decode to reject trailing
  garbage after a valid JSON object (prevents silent acceptance of malformed
  input like '{"name":"x"}garbage')
- review/persona_test.go: add TestJSONTrailingContentRejected test

NIT fixes:
- review/persona.go: add default case to checkYAMLDepth switch with
  explanatory comment about scalar leaf nodes
- review/persona.go: document AnchorNode depth+1 conservative asymmetry
- review/persona.go: simplify redundant if-guard in ListBuiltinPersonas

review/persona_test.go

@@ -858,3 +858,41 @@ identity: test identity
 		t.Errorf("Name = %q, want %q", p.Name, "test")
 	}
 }
+
+func TestJSONTrailingContentRejected(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+	}{
+		{
+			name:    "trailing garbage after object",
+			content: `{"name":"test","identity":"test identity"}garbage`,
+		},
+		{
+			name:    "two JSON objects",
+			content: `{"name":"test","identity":"test identity"}{"name":"other"}`,
+		},
+		{
+			name:    "trailing array",
+			content: `{"name":"test","identity":"test identity"}[]`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dir := t.TempDir()
+			path := filepath.Join(dir, "test.json")
+			if err := os.WriteFile(path, []byte(tt.content), 0644); err != nil {
+				t.Fatalf("failed to write test file: %v", err)
+			}
+
+			_, err := LoadPersona(path)
+			if err == nil {
+				t.Fatal("expected error for trailing content, got nil")
+			}
+			if !strings.Contains(err.Error(), "trailing content") {
+				t.Errorf("error = %q, want to contain 'trailing content'", err.Error())
+			}
+		})
+	}
+}