revert: remove YAML support, keep JSON-only

The external dependency (goccy/go-yaml) violates the repository's
stdlib-only convention (CONVENTIONS.md). While YAML provides better
readability for multi-line strings, the convenience doesn't justify
breaking a hard rule.

Reverts:
- External dependency on github.com/goccy/go-yaml
- YAML parsing logic in persona.go
- YAML persona files (restored as JSON)
- YAML-specific tests
- Design document (feature rejected)

The persona files work fine with JSON. Multi-line strings use \n escapes
which is less pretty but acceptable for internal files.

This addresses all MAJOR findings from review bots regarding the external
dependency violation.

review/persona.go

@@ -7,35 +7,32 @@ import (
 	"os"
 	"strings"
 	"unicode/utf8"
-
-	"github.com/goccy/go-yaml"
 )
 
-//go:embed personas/*.yaml
+//go:embed personas/*.json
 var embeddedPersonas embed.FS
 
 // Persona defines a specialized review role with focused expertise.
 type Persona struct {
-	Name         string   `json:"name" yaml:"name"`
-	DisplayName  string   `json:"display_name" yaml:"display_name"`
-	ModelPref    string   `json:"model_preference,omitempty" yaml:"model_preference,omitempty"`
-	Identity     string   `json:"identity" yaml:"identity"`
-	Focus        []string `json:"focus" yaml:"focus"`
-	Ignore       []string `json:"ignore" yaml:"ignore"`
-	Severity     Severity `json:"severity" yaml:"severity"`
-	OutputFormat string   `json:"output_format,omitempty" yaml:"output_format,omitempty"`
+	Name         string   `json:"name"`
+	DisplayName  string   `json:"display_name"`
+	ModelPref    string   `json:"model_preference,omitempty"`
+	Identity     string   `json:"identity"`
+	Focus        []string `json:"focus"`
+	Ignore       []string `json:"ignore"`
+	Severity     Severity `json:"severity"`
+	OutputFormat string   `json:"output_format,omitempty"`
 }
 
 // Severity defines what constitutes each severity level for this persona.
 // These are prompt guidance for the LLM, not output format changes.
 type Severity struct {
-	Major string `json:"major" yaml:"major"`
-	Minor string `json:"minor" yaml:"minor"`
-	Nit   string `json:"nit" yaml:"nit"`
+	Major string `json:"major"`
+	Minor string `json:"minor"`
+	Nit   string `json:"nit"`
 }
 
-// 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.
+// LoadPersona loads a persona from a JSON file path.
 func LoadPersona(path string) (*Persona, error) {
 	data, err := os.ReadFile(path)
 	if err != nil {
@@ -46,23 +43,14 @@ func LoadPersona(path string) (*Persona, error) {
 
 // LoadBuiltinPersona loads a built-in persona by name.
 // Returns an error if the persona doesn't exist.
-// Built-in personas are stored in YAML format.
 func LoadBuiltinPersona(name string) (*Persona, error) {
-	// Try YAML first (preferred format)
-	yamlFile := name + ".yaml"
-	data, err := embeddedPersonas.ReadFile("personas/" + yamlFile)
-	if err == nil {
-		return parsePersona(data, "builtin:"+yamlFile)
-	}
-
-	// Fall back to JSON for backwards compatibility
-	jsonFile := name + ".json"
-	data, err = embeddedPersonas.ReadFile("personas/" + jsonFile)
+	filename := name + ".json"
+	data, err := embeddedPersonas.ReadFile("personas/" + filename) // embed.FS paths use forward slashes per io/fs spec
 	if err != nil {
 		available := ListBuiltinPersonas()
 		return nil, fmt.Errorf("unknown built-in persona %q (available: %s)", name, strings.Join(available, ", "))
 	}
-	return parsePersona(data, "builtin:"+jsonFile)
+	return parsePersona(data, "builtin:"+name)
 }
 
 // ListBuiltinPersonas returns the names of all built-in personas.
@@ -72,50 +60,22 @@ func ListBuiltinPersonas() []string {
 	if err != nil {
 		return []string{}
 	}
-	seen := make(map[string]bool)
+	var names []string
 	for _, e := range entries {
 		if e.IsDir() {
 			continue
 		}
 		name := e.Name()
-		// Strip extension to get persona name
-		var personaName string
-		switch {
-		case strings.HasSuffix(name, ".yaml"):
-			personaName = strings.TrimSuffix(name, ".yaml")
-		case strings.HasSuffix(name, ".yml"):
-			personaName = strings.TrimSuffix(name, ".yml")
-		case strings.HasSuffix(name, ".json"):
-			personaName = strings.TrimSuffix(name, ".json")
-		default:
-			continue
+		if strings.HasSuffix(name, ".json") {
+			names = append(names, strings.TrimSuffix(name, ".json"))
 		}
-		if !seen[personaName] {
-			seen[personaName] = true
-		}
-	}
-	var names []string
-	for name := range seen {
-		names = append(names, name)
 	}
 	return names
 }
 
-// parsePersona parses persona data from JSON or YAML format.
-// Format is detected by the source file extension.
 func parsePersona(data []byte, source string) (*Persona, error) {
-	lowerSource := strings.ToLower(source)
-	isYAML := strings.HasSuffix(lowerSource, ".yaml") || strings.HasSuffix(lowerSource, ".yml")
-
 	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)
-	} else {
-		err = json.Unmarshal(data, &p)
-	}
-	if err != nil {
+	if err := json.Unmarshal(data, &p); err != nil {
 		return nil, fmt.Errorf("parse persona %s: %w", source, err)
 	}
 	if err := validatePersona(&p, source); err != nil {