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/container-volumes/push.yml

@@ -0,0 +1,19 @@
+name: Job Container
+on: push
+
+jobs:
+  with-volumes:
+    runs-on: ubuntu-latest
+    container:
+      image: node:24-bookworm-slim
+      volumes:
+        - my_docker_volume:/path/to/volume
+        - /path/to/nonexist/directory
+        - /proc/sys/kernel/random/boot_id:/current/boot_id
+    steps:
+      - run: |
+          set -e
+          test -d /path/to/volume
+          test "$(cat /proc/sys/kernel/random/boot_id)" = "$(cat /current/boot_id)"
+          test -d /path/to/nonexist/directory
+