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/push.yaml

@@ -0,0 +1,23 @@
+name: services
+on: push
+jobs:
+  services:
+    name: Reproduction of failing Services interpolation
+    runs-on: ubuntu-latest
+    services:
+      postgres:
+        image: nginx:alpine
+        ports:
+          - 80
+    steps:
+      - name: Echo the Postgres service ID / Network / Ports
+        run: |
+          echo "id: ${{ job.services.postgres.id }}"
+          echo "network: ${{ job.services.postgres.network }}"
+          echo "ports: ${{ job.services.postgres.ports }}"
+      - name: The job context describes the started containers
+        run: |
+          test -n "${{ job.container.id }}"
+          test -n "${{ job.services.postgres.id }}"
+          test -n "${{ job.services.postgres.ports['80'] }}"
+          test "${{ job.services.postgres.network }}" = "${{ job.container.network }}"