fix: address remaining PR #58 review findings

1. Remove dead JSON fallback in LoadBuiltinPersona
   - The embed directive only includes *.yaml files
   - JSON fallback code could never succeed
   - Simplified function to only try YAML

2. JSON parsing now rejects unknown fields
   - Switched from json.Unmarshal to json.Decoder
   - DisallowUnknownFields() matches YAML's KnownFields(true)
   - Added test coverage for JSON unknown field rejection

3. Documented symlink support in LoadPersona
   - os.Stat follows symlinks, so symlinks to regular files work
   - Added doc comment explaining the behavior
   - Added test for symlink support

review/persona.go

@@ -51,7 +51,13 @@ 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.
+//
+// Symlinks are supported: os.Stat follows symlinks, so a symlink pointing to
+// a regular file will pass the IsRegular() check. Symlinks to non-regular files
+// (directories, FIFOs, devices) are still rejected.
 func LoadPersona(path string) (*Persona, error) {
+	// os.Stat follows symlinks, so symlinks to regular files are supported.
+	// The IsRegular() check operates on the target, not the symlink itself.
 	info, err := os.Stat(path)
 	if err != nil {
 		return nil, fmt.Errorf("read persona file %s: %w", path, err)
@@ -76,23 +82,15 @@ 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.
+// Built-in personas are stored in YAML format only (see embed directive).
 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)
 	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:"+yamlFile)
 }
 
 // ListBuiltinPersonas returns the names of all built-in personas in sorted order.
@@ -143,7 +141,11 @@ func parsePersona(data []byte, source string) (*Persona, error) {
 	if isYAML {
 		err = unmarshalYAMLWithDepthLimit(data, &p, MaxYAMLDepth)
 	} else {
-		err = json.Unmarshal(data, &p)
+		// Use json.Decoder with DisallowUnknownFields for consistency with
+		// YAML's KnownFields(true) - both reject unknown fields to catch typos.
+		dec := json.NewDecoder(bytes.NewReader(data))
+		dec.DisallowUnknownFields()
+		err = dec.Decode(&p)
 	}
 	if err != nil {
 		return nil, fmt.Errorf("parse persona %s: %w", source, err)