fix: use build tags for Setpgid (Unix-only) to fix Windows build

pkg/hooks/procattr_unix.go

@@ -0,0 +1,12 @@
+//go:build !windows
+
+package hooks
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+func setSysProcAttr(cmd *exec.Cmd) {
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+}

pkg/hooks/procattr_windows.go

@@ -0,0 +1,9 @@
+//go:build windows
+
+package hooks
+
+import "os/exec"
+
+func setSysProcAttr(cmd *exec.Cmd) {
+	// Windows has no Setpgid; process groups work differently.
+}

pkg/hooks/worker.go

@@ -334,7 +334,7 @@ func ensureWorkerRunningCtx(ctx context.Context) (int, error) {
 	// Start worker -- detach from hook's process group so Claude Code
 	// killing the hook doesn't take the worker down with it.
 	cmd := exec.Command(workerPath) // #nosec G204 -- workerPath is from internal findWorkerBinary
-	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	setSysProcAttr(cmd)
 	cmd.Stdout = os.Stderr
 	cmd.Stderr = os.Stderr
 	if err := cmd.Start(); err != nil {