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-environment-and-context-tests/js/index.js

@@ -0,0 +1,15 @@
+function checkEnvVar({ name, allowEmpty }) {
+  if (
+    process.env[name] === undefined ||
+    (allowEmpty === false && process.env[name] === "")
+  ) {
+    throw new Error(
+      `${name} is undefined` + (allowEmpty === false ? " or empty" : "")
+    );
+  }
+  console.log(`${name}=${process.env[name]}`);
+}
+
+checkEnvVar({ name: "GITHUB_ACTION", allowEmpty: false });
+checkEnvVar({ name: "GITHUB_ACTION_REPOSITORY", allowEmpty: true /* allows to be empty for local actions */ });
+checkEnvVar({ name: "GITHUB_ACTION_REF", allowEmpty: true /* allows to be empty for local actions */ });