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/uses-docker-url/push.yml

@@ -0,0 +1,23 @@
+name: uses-docker-url
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: docker://node:24-bookworm-slim
+        with:
+          somekey: somevalue
+          args: echo ${INPUT_SOMEKEY} | grep somevalue
+      - uses: docker://node:24-bookworm-slim
+        with:
+          args: -v
+      - uses: docker://node:24-bookworm-slim
+        with:
+          entrypoint: /bin/sh
+          args: -c -- uname -a
+      - uses: docker://node:24-bookworm-slim
+        with:
+          entrypoint: /bin/sh
+          args: -c 'uname -a'
+