fix(review): address review feedback on persona YAML handling

- Reorder empty doc check before multi-doc check for natural flow
- Detect nil-body docs (whitespace-only, comment-only input)
- Add explanatory comment on pointer identity for cycle detection map
- Improve depth-counting test comment with AST walker specifics
- Add TestYAMLEmptyFileRejection covering empty/whitespace/comment inputs

Addresses MINOR and NIT findings from sonnet, gpt, and security reviews.
MAJOR (allowlist violation) tracked in issue #91.

review/persona_test.go

@@ -459,7 +459,8 @@ func TestYAMLDeeplyNestedRejection(t *testing.T) {
 	path := filepath.Join(dir, "deeply-nested.yaml")
 
 	// Build a deeply nested YAML structure that exceeds MaxYAMLDepth (20).
-	// Each level adds to the depth count via mapping values.
+	// Each nested mapping key generates a MappingValueNode, incrementing depth
+	// by 1 per level in the AST walk. With 25 levels, we exceed MaxYAMLDepth (20).
 	var sb strings.Builder
 	sb.WriteString("name: test\nidentity: test\nnested:\n")
 	indent := "  "
@@ -483,6 +484,37 @@ func TestYAMLDeeplyNestedRejection(t *testing.T) {
 	}
 }
 
+
+func TestYAMLEmptyFileRejection(t *testing.T) {
+	dir := t.TempDir()
+
+	tests := []struct {
+		name    string
+		content string
+	}{
+		{"completely empty", ""},
+		{"whitespace only", "   \n\n  "},
+		{"comment only", "# just a comment\n"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			path := filepath.Join(dir, tc.name+".yaml")
+			if err := os.WriteFile(path, []byte(tc.content), 0644); err != nil {
+				t.Fatalf("failed to write test file: %v", err)
+			}
+
+			_, err := LoadPersona(path)
+			if err == nil {
+				t.Error("expected error for empty YAML input, got nil")
+			}
+			if err != nil && !strings.Contains(err.Error(), "empty YAML document") {
+				t.Errorf("expected error containing %q, got: %v", "empty YAML document", err)
+			}
+		})
+	}
+}
+
 func TestYAMLFileSizeLimit(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "huge.yaml")