docs: update DESIGN-57 to reflect goccy/go-yaml as the supported YAML library

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 `gopkg.in/yaml.v3` (approved in CONVENTIONS.md) with explicit depth limiting
+- Library: use `github.com/goccy/go-yaml` v1.16.0+ (approved in CONVENTIONS.md); has built-in depth protection via `MaxYAMLDepth`/`MaxYAMLNodes` constants
 
 ## Proposed Approach
 
@@ -63,7 +63,7 @@ func checkYAMLDepth(node *yaml.Node, depth, maxDepth int) error {
 }
 ```
 
-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 (including alias resolution), then decoding into the target struct.
+The `github.com/goccy/go-yaml` library provides built-in depth protection via `MaxYAMLDepth` and `MaxYAMLNodes` decoder options. We use these instead of a manual depth-checking walk.
 
 ## State/Data Model