fix: address review feedback on persona feature

MAJOR fixes:
- Remove external YAML dependency (github.com/goccy/go-yaml)
  Per project convention: Go standard library only, zero dependencies.
  Convert all persona files from YAML to JSON format.
- Fix TestValidateWorkspacePath error expectation
  Go 1.21+ filepath.Join normalizes absolute paths differently.

MINOR fixes:
- Remove custom contains helper in persona_test.go (use strings.Contains)
- Add Unicode-safe CapitalizeFirst function for header titles
- ListBuiltinPersonas returns empty slice instead of nil on error
- Fix test comment about filepath.Join behavior

Documentation:
- Update README to reflect JSON-only persona format
- Update design doc with note about JSON decision
- Fix action.yml description for persona-file input

review/persona.go

@@ -5,37 +5,34 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
-	"path/filepath"
 	"strings"
-
-	"github.com/goccy/go-yaml"
+	"unicode/utf8"
 )
 
-//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 file path.
-// Supports both YAML (.yaml, .yml) and JSON (.json) formats.
+// LoadPersona loads a persona from a JSON file path.
 func LoadPersona(path string) (*Persona, error) {
 	data, err := os.ReadFile(path)
 	if err != nil {
@@ -47,7 +44,7 @@ func LoadPersona(path string) (*Persona, error) {
 // LoadBuiltinPersona loads a built-in persona by name.
 // Returns an error if the persona doesn't exist.
 func LoadBuiltinPersona(name string) (*Persona, error) {
-	filename := name + ".yaml"
+	filename := name + ".json"
 	data, err := embeddedPersonas.ReadFile("personas/" + filename) // embed.FS paths use forward slashes per io/fs spec
 	if err != nil {
 		available := ListBuiltinPersonas()
@@ -57,10 +54,11 @@ func LoadBuiltinPersona(name string) (*Persona, error) {
 }
 
 // ListBuiltinPersonas returns the names of all built-in personas.
+// Returns an empty slice if the embedded directory cannot be read.
 func ListBuiltinPersonas() []string {
 	entries, err := embeddedPersonas.ReadDir("personas")
 	if err != nil {
-		return nil
+		return []string{}
 	}
 	var names []string
 	for _, e := range entries {
@@ -68,10 +66,8 @@ func ListBuiltinPersonas() []string {
 			continue
 		}
 		name := e.Name()
-		if strings.HasSuffix(name, ".yaml") {
-			names = append(names, strings.TrimSuffix(name, ".yaml"))
-		} else if strings.HasSuffix(name, ".yml") {
-			names = append(names, strings.TrimSuffix(name, ".yml"))
+		if strings.HasSuffix(name, ".json") {
+			names = append(names, strings.TrimSuffix(name, ".json"))
 		}
 	}
 	return names
@@ -79,20 +75,9 @@ func ListBuiltinPersonas() []string {
 
 func parsePersona(data []byte, source string) (*Persona, error) {
 	var p Persona
-	
-	// Determine format by extension or try YAML first (it's a superset of JSON)
-	ext := strings.ToLower(filepath.Ext(source))
-	if ext == ".json" {
-		if err := json.Unmarshal(data, &p); err != nil {
-			return nil, fmt.Errorf("parse persona %s: %w", source, err)
-		}
-	} else {
-		// YAML (also handles .yaml, .yml, and builtin: prefix)
-		if err := yaml.Unmarshal(data, &p); err != nil {
-			return nil, fmt.Errorf("parse persona %s: %w", source, err)
-		}
+	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 {
 		return nil, err
 	}
@@ -112,3 +97,16 @@ func validatePersona(p *Persona, source string) error {
 	}
 	return nil
 }
+
+// CapitalizeFirst capitalizes the first rune of a string in a Unicode-safe way.
+// Returns the original string if it's empty.
+func CapitalizeFirst(s string) string {
+	if s == "" {
+		return s
+	}
+	r, size := utf8.DecodeRuneInString(s)
+	if r == utf8.RuneError {
+		return s
+	}
+	return strings.ToUpper(string(r)) + s[size:]
+}