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/networking/push.yml

@@ -0,0 +1,14 @@
+name: test network setup
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install tools
+        run: |
+          apt update
+          apt install -y iputils-ping
+      - name: Run hostname test
+        run: |
+          hostname -f
+          ping -c 4 $(hostname -f)