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/docker-action-custom-path/push.yml

@@ -0,0 +1,12 @@
+on: push
+jobs:
+  _:
+    runs-on: ubuntu-latest
+    steps:
+    - run: |
+        FROM node:24-bookworm-slim
+        ENV PATH="/opt/texlive/texdir/bin/x86_64-linuxmusl:${PATH}"
+        ENV ORG_PATH="${PATH}"
+        ENTRYPOINT [ "bash", "-c", "echo \"PATH=$PATH\" && echo \"ORG_PATH=$ORG_PATH\" && [[ \"$PATH\" = \"$ORG_PATH\" ]]" ]
+      shell: mv {0} Dockerfile
+    - uses: ./