Formula cycle detection: show full extends chain in error message (gt-8tmz.15)

When bd cook encounters a circular extends chain (A extends B extends A),
the error message now shows the full chain: "cycle-a -> cycle-b -> cycle-a"
instead of just "circular extends detected: cycle-a".

This makes debugging circular dependencies much easier by showing exactly
which formulas are involved in the cycle.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/parser_test.go

@@ -3,6 +3,7 @@ package formula
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -533,6 +534,18 @@ func TestResolve_CircularExtends(t *testing.T) {
 	if err == nil {
 		t.Error("Resolve should fail for circular extends")
 	}
+
+	// Verify the error message shows the full cycle chain
+	errStr := err.Error()
+	if !strings.Contains(errStr, "cycle-a") {
+		t.Errorf("error should mention cycle-a: %v", err)
+	}
+	if !strings.Contains(errStr, "cycle-b") {
+		t.Errorf("error should mention cycle-b: %v", err)
+	}
+	if !strings.Contains(errStr, "->") {
+		t.Errorf("error should show cycle chain with '->': %v", err)
+	}
 }
 
 func TestGetStepByID(t *testing.T) {