feat(persona): add role-based review personas

Add persona system for specialized review roles. Each persona defines:
- A specific review focus (security, architecture, documentation)
- Custom system prompt additions
- Personality/tone adjustments

Built-in personas: security, architect, docs
Custom personas: load from JSON via persona-file flag

Includes workspace validation to prevent path traversal attacks.

Closes #51

review/persona.go

@@ -0,0 +1,114 @@
+package review
+
+import (
+	"embed"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/goccy/go-yaml"
+)
+
+//go:embed personas/*.yaml
+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"`
+}
+
+// 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"`
+}
+
+// LoadPersona loads a persona from a file path.
+// Supports both YAML (.yaml, .yml) and JSON (.json) formats.
+func LoadPersona(path string) (*Persona, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read persona file %s: %w", path, err)
+	}
+	return parsePersona(data, path)
+}
+
+// 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"
+	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:"+name)
+}
+
+// ListBuiltinPersonas returns the names of all built-in personas.
+func ListBuiltinPersonas() []string {
+	entries, err := embeddedPersonas.ReadDir("personas")
+	if err != nil {
+		return nil
+	}
+	var names []string
+	for _, e := range entries {
+		if e.IsDir() {
+			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"))
+		}
+	}
+	return names
+}
+
+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 := validatePersona(&p, source); err != nil {
+		return nil, err
+	}
+	return &p, nil
+}
+
+func validatePersona(p *Persona, source string) error {
+	if p.Name == "" {
+		return fmt.Errorf("persona %s: name is required", source)
+	}
+	if p.Identity == "" {
+		return fmt.Errorf("persona %s: identity is required", source)
+	}
+	// DisplayName defaults to Name if not set
+	if p.DisplayName == "" {
+		p.DisplayName = p.Name
+	}
+	return nil
+}