refactor: move act under internal

The runner is an application, not a library. `act/model` and
`act/exprparser` were the last packages anything outside this repository
consumed and they now live in actionslib, so nothing needs the rest of
`act` to be importable, and keeping it importable invites the coupling
that was just removed.

Import paths only, the files are unchanged.

Assisted-by: Codet:GPT-5.1-Codex

internal/act/container/util.go

@@ -0,0 +1,17 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Copyright 2022 The nektos/act Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build (!windows && !plan9 && !openbsd) || (!windows && !plan9 && !mips64)
+
+package container
+
+import (
+	"os"
+
+	"github.com/creack/pty"
+)
+
+func openPty() (*os.File, *os.File, error) {
+	return pty.Open()
+}