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/node24/index.js

@@ -0,0 +1,14 @@
+import {appendFileSync, readFileSync} from 'node:fs';
+
+const nameToGreet = process.env['INPUT_WHO-TO-GREET'] || 'World';
+console.log(`Hello ${nameToGreet}!`);
+
+if (process.env.GITHUB_OUTPUT) {
+  appendFileSync(process.env.GITHUB_OUTPUT, `time=${new Date().toTimeString()}\n`);
+}
+
+let payload = {};
+if (process.env.GITHUB_EVENT_PATH) {
+  payload = JSON.parse(readFileSync(process.env.GITHUB_EVENT_PATH, 'utf8'));
+}
+console.log(`The event payload: ${JSON.stringify(payload, undefined, 2)}`);