fix: handle MergeKeyNode explicitly in depth check, add size limit to ParsePersonaBytes

- Add explicit case for *ast.MergeKeyNode in checkYAMLDepth switch to
  make it clear this is an intentional leaf (no children to recurse)
  rather than relying on the default case. Prevents future library
  changes from silently bypassing depth checks.

- Add MaxPersonaFileSize bound check at the top of ParsePersonaBytes.
  While callers already check size, the public API should defend itself
  (defense in depth) against arbitrarily large inputs that could cause
  excessive memory/CPU before AST validation runs.

- Add tests for both behaviors.

Addresses review #2879 findings.

review/persona.go

@@ -307,10 +307,16 @@ func checkYAMLDepth(node ast.Node, depth, maxDepth, maxNodes int, validated map[
 		if err := checkYAMLDepth(n.Value, depth+1, maxDepth, maxNodes, validated, visiting, nodeCount); err != nil {
 			return err
 		}
+	case *ast.MergeKeyNode:
+		// MergeKeyNode represents the literal "<<" merge key token. It has no
+		// child nodes — the value side of a merge (e.g., *alias) lives in the
+		// parent MappingValueNode.Value, which is already recursed into above.
+		// Explicitly listed here (rather than in the default case) to prevent
+		// future library changes from silently bypassing depth checks.
 	default:
 		// Scalar leaf nodes (StringNode, IntegerNode, FloatNode, BoolNode,
-		// NullNode, InfinityNode, NanNode, LiteralNode, MergeKeyNode) have
-		// no children to recurse into.
+		// NullNode, InfinityNode, NanNode, LiteralNode) have no children to
+		// recurse into.
 	}
 	return nil
 }
@@ -318,7 +324,11 @@ func checkYAMLDepth(node ast.Node, depth, maxDepth, maxNodes int, validated map[
 // ParsePersonaBytes parses persona data from bytes with a source label for errors.
 // This is useful for parsing personas fetched from external sources (e.g., Gitea API)
 // without requiring filesystem access. Format is detected by source extension.
+// Input is bounded by MaxPersonaFileSize to prevent resource exhaustion.
 func ParsePersonaBytes(data []byte, source string) (*Persona, error) {
+	if len(data) > MaxPersonaFileSize {
+		return nil, fmt.Errorf("persona data from %s exceeds maximum size (%d bytes, limit %d)", source, len(data), MaxPersonaFileSize)
+	}
 	return parsePersona(data, source)
 }