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/runner/testdata/actions/script/post.js

@@ -0,0 +1,6 @@
+import {execFileSync} from 'node:child_process';
+
+const script = process.env.INPUT_POST;
+if (script) {
+  execFileSync('bash', ['-eo', 'pipefail', '-c', script], {stdio: 'inherit'});
+}