fix: address PR review findings

MAJOR fixes:
- Remove false security claim about gopkg.in/yaml.v3 having built-in depth protection
- Add explicit YAML depth limiting via yaml.Node API (MaxYAMLDepth=20)
- Add file size limit for persona files (MaxPersonaFileSize=64KB)
- Add test for deeply nested YAML rejection

MINOR fixes:
- Add sort.Strings to ListBuiltinPersonas for deterministic ordering
- Update design doc to reflect actual library used (gopkg.in/yaml.v3)
- Update README: 'Zero dependencies' → 'Minimal dependencies'
- Add test for file size limit
- Add test for sorted persona list

review/persona.go

@@ -1,10 +1,12 @@
 package review
 
 import (
+	"bytes"
 	"embed"
 	"encoding/json"
 	"fmt"
 	"os"
+	"sort"
 	"strings"
 	"unicode/utf8"
 
@@ -14,6 +16,14 @@ import (
 //go:embed personas/*.yaml
 var embeddedPersonas embed.FS
 
+// MaxPersonaFileSize is the maximum size for persona files (64 KB).
+// This prevents denial-of-service via excessively large files.
+const MaxPersonaFileSize = 64 * 1024
+
+// MaxYAMLDepth is the maximum nesting depth allowed in YAML persona files.
+// This prevents stack exhaustion from deeply nested structures.
+const MaxYAMLDepth = 20
+
 // Persona defines a specialized review role with focused expertise.
 type Persona struct {
 	Name         string   `json:"name" yaml:"name"`
@@ -36,7 +46,15 @@ type Severity struct {
 
 // LoadPersona loads a persona from a JSON or YAML file path.
 // Format is detected by file extension: .yaml/.yml for YAML, .json or other for JSON.
+// Files larger than MaxPersonaFileSize are rejected.
 func LoadPersona(path string) (*Persona, error) {
+	info, err := os.Stat(path)
+	if err != nil {
+		return nil, fmt.Errorf("read persona file %s: %w", path, err)
+	}
+	if info.Size() > MaxPersonaFileSize {
+		return nil, fmt.Errorf("persona file %s exceeds maximum size (%d bytes)", path, MaxPersonaFileSize)
+	}
 	data, err := os.ReadFile(path)
 	if err != nil {
 		return nil, fmt.Errorf("read persona file %s: %w", path, err)
@@ -65,7 +83,7 @@ func LoadBuiltinPersona(name string) (*Persona, error) {
 	return parsePersona(data, "builtin:"+jsonFile)
 }
 
-// ListBuiltinPersonas returns the names of all built-in personas.
+// ListBuiltinPersonas returns the names of all built-in personas in sorted order.
 // Returns an empty slice if the embedded directory cannot be read.
 func ListBuiltinPersonas() []string {
 	entries, err := embeddedPersonas.ReadDir("personas")
@@ -94,10 +112,11 @@ func ListBuiltinPersonas() []string {
 			seen[personaName] = true
 		}
 	}
-	var names []string
+	names := make([]string, 0, len(seen))
 	for name := range seen {
 		names = append(names, name)
 	}
+	sort.Strings(names)
 	return names
 }
 
@@ -110,8 +129,7 @@ func parsePersona(data []byte, source string) (*Persona, error) {
 	var p Persona
 	var err error
 	if isYAML {
-		// go-yaml v1.16.0+ has built-in protection against deeply nested structures
-		err = yaml.Unmarshal(data, &p)
+		err = unmarshalYAMLWithDepthLimit(data, &p, MaxYAMLDepth)
 	} else {
 		err = json.Unmarshal(data, &p)
 	}
@@ -124,6 +142,35 @@ func parsePersona(data []byte, source string) (*Persona, error) {
 	return &p, nil
 }
 
+// unmarshalYAMLWithDepthLimit unmarshals YAML data with explicit depth limiting.
+// This protects against stack exhaustion from deeply nested structures.
+func unmarshalYAMLWithDepthLimit(data []byte, out interface{}, maxDepth int) error {
+	var node yaml.Node
+	dec := yaml.NewDecoder(bytes.NewReader(data))
+	if err := dec.Decode(&node); err != nil {
+		return err
+	}
+
+	if err := checkYAMLDepth(&node, 0, maxDepth); err != nil {
+		return err
+	}
+
+	return node.Decode(out)
+}
+
+// checkYAMLDepth recursively checks that YAML nodes don't exceed the depth limit.
+func checkYAMLDepth(node *yaml.Node, depth, maxDepth int) error {
+	if depth > maxDepth {
+		return fmt.Errorf("YAML nesting depth exceeds maximum (%d)", maxDepth)
+	}
+	for _, child := range node.Content {
+		if err := checkYAMLDepth(child, depth+1, maxDepth); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func validatePersona(p *Persona, source string) error {
 	if p.Name == "" {
 		return fmt.Errorf("persona %s: name is required", source)