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/actions/docker-url/action.yml

@@ -0,0 +1,16 @@
+name: docker-url
+author: nektos
+description: testing
+inputs:
+  who-to-greet:
+    description: who to greet
+    required: true
+    default: World
+runs:
+  using: docker
+  image: docker://node:24-bookworm-slim
+  entrypoint: /bin/sh -c
+  env:
+    TEST: enabled
+  args:
+    - env