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/checkout/push.yml

@@ -0,0 +1,8 @@
+name: checkout
+on: push
+
+jobs:
+  checkout:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2