fix(review): address review 2792 feedback

- Document nodeCount overcounting as intentional conservative behavior
  (bounds total validation work, not unique nodes)
- Improve TestYAMLDeeplyNestedRejection comment with concrete depth trace
- Replace outdated gopkg.in/yaml.v3 pseudocode in design doc with
  reference to authoritative implementation
- Update PR description to clarify pre-approval via issue #57

docs/DESIGN-57-yaml-persona.md

@@ -33,42 +33,16 @@ func parsePersona(data []byte, source string) (*Persona, error) {
 
 ### YAML Parsing with Depth Protection
 
-> **Note:** The pseudocode below reflects the initial design using `gopkg.in/yaml.v3`
-> types (`yaml.Node`). The actual implementation uses `github.com/goccy/go-yaml`
-> with `ast.Node`-based traversal, dual-map cycle/depth tracking, and node-count
-> limits. See `review/persona.go` for the current implementation.
+We implement a custom AST-based depth/node-count walk (`checkYAMLDepth` in
+`review/persona.go`) rather than relying on library decoder options. Key design
+decisions:
 
-```go
-func unmarshalYAMLWithDepthLimit(data []byte, out any, maxDepth int) error {
-    var node yaml.Node
-    dec := yaml.NewDecoder(bytes.NewReader(data))
-    if err := dec.Decode(&node); err != nil {
-        return err
-    }
-    if err := checkYAMLDepth(&node, 0, maxDepth); err != nil {
-        return err
-    }
-    return node.Decode(out)
-}
+- **Library:** `github.com/goccy/go-yaml` with `ast.Node`-based traversal
+- **Dual-map tracking:** `validated` (depth-aware short-circuit) + `visiting` (cycle detection)
+- **Node-count limit:** Conservative overcounting bounds total validation work
+- **Alias-aware depth:** Aliases increment depth and are re-checked when encountered at greater depths
 
-func checkYAMLDepth(node *yaml.Node, depth, maxDepth int) error {
-    if depth > maxDepth {
-        return fmt.Errorf("YAML nesting depth exceeds maximum (%d)", maxDepth)
-    }
-    // Handle alias nodes by following the Alias pointer
-    if node.Kind == yaml.AliasNode && node.Alias != nil {
-        return checkYAMLDepth(node.Alias, depth, maxDepth)
-    }
-    for _, child := range node.Content {
-        if err := checkYAMLDepth(child, depth+1, maxDepth); err != nil {
-            return err
-        }
-    }
-    return nil
-}
-```
-
-We implement a custom AST-based depth/node-count walk (`checkYAMLDepth`) rather than relying on library decoder options. This gives us precise control over how depth is counted across aliases and anchors, with a depth-aware validated map to prevent alias depth bypass.
+See `review/persona.go:checkYAMLDepth` for the authoritative implementation.
 
 ## State/Data Model