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

docs/DESIGN-57-yaml-persona.md

@@ -9,7 +9,7 @@ JSON is awkward for persona files that contain multi-line text (identity, severi
 - Backwards compatibility: existing JSON personas must continue to work
 - Security: protect against DoS via deeply nested YAML (AIKIDO-2024-10486)
 - Consistency: use `.yaml` extension (not `.yml`)
-- Library: use `github.com/goccy/go-yaml` v1.16.0+ (actively maintained, security fix applied)
+- Library: use `gopkg.in/yaml.v3` (approved in CONVENTIONS.md) with explicit depth limiting
 
 ## Proposed Approach
 
@@ -36,8 +36,8 @@ func parsePersona(data []byte, source string) (*Persona, error) {
 ```go
 func parseYAML(data []byte, source string) (*Persona, error) {
     var p Persona
-    // go-yaml has built-in protection against deeply nested structures
-    // but we add explicit decoder options for defense in depth
+    // gopkg.in/yaml.v3 does NOT have built-in depth limiting.
+    // Use explicit depth check via yaml.Node API.
     if err := yaml.Unmarshal(data, &p); err != nil {
         return nil, fmt.Errorf("parse persona %s: %w", source, err)
     }
@@ -48,7 +48,7 @@ func parseYAML(data []byte, source string) (*Persona, error) {
 }
 ```
 
-The `goccy/go-yaml` library since v1.16.0 limits nesting depth by default.
+The `gopkg.in/yaml.v3` library does not have built-in depth protection, so we implement explicit depth checking by first decoding into a `yaml.Node`, walking the tree to verify depth, then decoding into the target struct.
 
 ## State/Data Model