fix(deps): replace gopkg.in/yaml.v3 with github.com/goccy/go-yaml

Fixes #87.

PR #58 incorrectly added gopkg.in/yaml.v3 (abandoned library) instead of
github.com/goccy/go-yaml as required by issue #57.

Changes:
- Replace gopkg.in/yaml.v3 with github.com/goccy/go-yaml v1.19.2
- Update review/persona.go to use goccy/go-yaml API:
  - parser.ParseBytes for AST-based depth/node count checking
  - yaml.Strict() decoder option instead of KnownFields(true)
  - ast.Node types instead of yaml.Node for tree walking
- Update review/persona_test.go to use ast types for cycle tests
- Remove gopkg.in/yaml.v3 from go.mod and go.sum

All existing YAML tests pass with the new library.

review/persona_test.go

@@ -7,7 +7,7 @@ import (
 	"strings"
 	"testing"
 
-	"gopkg.in/yaml.v3"
+	"github.com/goccy/go-yaml/ast"
 )
 
 func TestLoadBuiltinPersona(t *testing.T) {
@@ -459,7 +459,7 @@ func TestYAMLDeeplyNestedRejection(t *testing.T) {
 	path := filepath.Join(dir, "deeply-nested.yaml")
 
 	// Build a deeply nested YAML structure that exceeds MaxYAMLDepth (20).
-	// Each level adds 2 to the depth count (key + value mapping).
+	// Each level adds to the depth count via mapping values.
 	var sb strings.Builder
 	sb.WriteString("name: test\nidentity: test\nnested:\n")
 	indent := "  "
@@ -505,30 +505,29 @@ func TestYAMLFileSizeLimit(t *testing.T) {
 func TestYAMLAliasCycleDetection(t *testing.T) {
 	// Test that our checkYAMLDepth function handles alias cycles gracefully
 	// by using the seen map to prevent infinite recursion.
-	// We test this directly because go-yaml's parser handles most cycles
-	// at parse time, but we need to ensure our checker is robust.
 
 	// Create a node structure where an alias points to a parent node,
-	// simulating what could happen with malicious input that bypasses
-	// go-yaml's cycle detection.
-	parent := &yaml.Node{
-		Kind: yaml.MappingNode,
-		Content: []*yaml.Node{
-			{Kind: yaml.ScalarNode, Value: "name"},
-			{Kind: yaml.ScalarNode, Value: "test"},
-			{Kind: yaml.ScalarNode, Value: "nested"},
+	// simulating what could happen with crafted input.
+	parent := &ast.MappingNode{
+		Values: []*ast.MappingValueNode{
+			{
+				Key:   &ast.StringNode{Value: "name"},
+				Value: &ast.StringNode{Value: "test"},
+			},
 		},
 	}
 
 	// Create a child that aliases back to the parent (artificial cycle)
-	aliasToParent := &yaml.Node{
-		Kind:  yaml.AliasNode,
-		Alias: parent,
+	aliasToParent := &ast.AliasNode{
+		Value: parent,
 	}
-	parent.Content = append(parent.Content, aliasToParent)
+	parent.Values = append(parent.Values, &ast.MappingValueNode{
+		Key:   &ast.StringNode{Value: "nested"},
+		Value: aliasToParent,
+	})
 
 	nodeCount := 0
-	seen := make(map[*yaml.Node]struct{})
+	seen := make(map[ast.Node]struct{})
 
 	// This should NOT hang or stack overflow - the seen map prevents infinite recursion
 	err := checkYAMLDepth(parent, 0, MaxYAMLDepth, MaxYAMLNodes, seen, &nodeCount)
@@ -594,27 +593,26 @@ func TestYAMLNodeCountLimit(t *testing.T) {
 func TestCheckYAMLDepthCycleDetectionDirect(t *testing.T) {
 	// Direct test of cycle detection in checkYAMLDepth by creating
 	// a node structure with an artificial cycle.
-	// This tests the seen map logic independent of go-yaml's parsing.
-	node := &yaml.Node{
-		Kind: yaml.MappingNode,
-		Content: []*yaml.Node{
-			{Kind: yaml.ScalarNode, Value: "key"},
-			{Kind: yaml.ScalarNode, Value: "value"},
+	node := &ast.MappingNode{
+		Values: []*ast.MappingValueNode{
+			{
+				Key:   &ast.StringNode{Value: "key"},
+				Value: &ast.StringNode{Value: "value"},
+			},
 		},
 	}
 
 	// Create a cycle by making a child reference the parent
-	cycleChild := &yaml.Node{
-		Kind:  yaml.AliasNode,
-		Alias: node, // Points back to the parent
+	cycleChild := &ast.AliasNode{
+		Value: node, // Points back to the parent
 	}
-	node.Content = append(node.Content,
-		&yaml.Node{Kind: yaml.ScalarNode, Value: "cyclic"},
-		cycleChild,
-	)
+	node.Values = append(node.Values, &ast.MappingValueNode{
+		Key:   &ast.StringNode{Value: "cyclic"},
+		Value: cycleChild,
+	})
 
 	nodeCount := 0
-	seen := make(map[*yaml.Node]struct{})
+	seen := make(map[ast.Node]struct{})
 	err := checkYAMLDepth(node, 0, MaxYAMLDepth, MaxYAMLNodes, seen, &nodeCount)
 
 	// Should complete without infinite recursion due to cycle detection