fix(review): address review feedback on persona YAML handling

- Reorder empty doc check before multi-doc check for natural flow
- Detect nil-body docs (whitespace-only, comment-only input)
- Add explanatory comment on pointer identity for cycle detection map
- Improve depth-counting test comment with AST walker specifics
- Add TestYAMLEmptyFileRejection covering empty/whitespace/comment inputs

Addresses MINOR and NIT findings from sonnet, gpt, and security reviews.
MAJOR (allowlist violation) tracked in issue #91.

review/persona.go

@@ -171,16 +171,18 @@ func unmarshalYAMLWithDepthLimit(data []byte, out any, maxDepth int) error {
 		return err
 	}
 
+	// Reject empty YAML input (whitespace-only, comment-only, or truly empty files).
+	// The parser returns a single doc with nil body for these cases.
+	if len(file.Docs) == 0 || file.Docs[0].Body == nil {
+		return fmt.Errorf("empty YAML document")
+	}
+
 	// Reject multi-document YAML files - silently ignoring additional documents
 	// could lead to confusing behavior where users think their changes take effect.
 	if len(file.Docs) > 1 {
 		return fmt.Errorf("multi-document YAML is not supported; only single-document files are allowed")
 	}
 
-	if len(file.Docs) == 0 {
-		return fmt.Errorf("empty YAML document")
-	}
-
 	nodeCount := 0
 	if err := checkYAMLDepth(file.Docs[0].Body, 0, maxDepth, MaxYAMLNodes, make(map[ast.Node]struct{}), &nodeCount); err != nil {
 		return err
@@ -210,7 +212,10 @@ func checkYAMLDepth(node ast.Node, depth, maxDepth, maxNodes int, seen map[ast.N
 		return fmt.Errorf("YAML node count exceeds maximum (%d)", maxNodes)
 	}
 
-	// Cycle detection: if we've seen this node before, we're in a cycle.
+	// Cycle detection: uses pointer identity (ast.Node is an interface, but all
+	// concrete node types are pointers) to detect revisits. This intentionally
+	// compares pointer identity, not structural equality, since we want to track
+	// specific node instances in the parsed AST graph.
 	if _, ok := seen[node]; ok {
 		return nil // Already validated this subtree, skip to avoid infinite recursion.
 	}