feat(signing): add Windows signtool placeholder

Placeholder for future Windows code signing support.

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

pkg/build/signing/signtool.go

@@ -0,0 +1,34 @@
+package signing
+
+import (
+	"context"
+)
+
+// WindowsSigner signs binaries using Windows signtool (placeholder).
+type WindowsSigner struct {
+	config WindowsConfig
+}
+
+// Compile-time interface check.
+var _ Signer = (*WindowsSigner)(nil)
+
+// NewWindowsSigner creates a new Windows signer.
+func NewWindowsSigner(cfg WindowsConfig) *WindowsSigner {
+	return &WindowsSigner{config: cfg}
+}
+
+// Name returns "signtool".
+func (s *WindowsSigner) Name() string {
+	return "signtool"
+}
+
+// Available returns false (not yet implemented).
+func (s *WindowsSigner) Available() bool {
+	return false
+}
+
+// Sign is a placeholder that does nothing.
+func (s *WindowsSigner) Sign(ctx context.Context, binary string) error {
+	// TODO: Implement Windows signing
+	return nil
+}