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/services-with-container/push.yml

@@ -0,0 +1,15 @@
+name: services-with-containers
+on: push
+jobs:
+  services-with-containers:
+    runs-on: ubuntu-latest
+    # https://docs.github.com/en/actions/using-containerized-services/about-service-containers#running-jobs-in-a-container
+    container:
+      image: "node:24-bookworm-slim"
+    services:
+      nginx:
+        image: "nginx:alpine"
+    steps:
+      - run: apt-get -qq update && apt-get -yqq install --no-install-recommends curl
+      # reach the service over the shared job network by its alias, no host port needed
+      - run: curl -v http://nginx:80