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/script/action.yml

@@ -0,0 +1,15 @@
+name: 'script'
+description: 'Run the shell scripts passed as inputs across the pre/main/post lifecycle'
+inputs:
+  main:
+    description: 'shell script to run in the main step'
+    required: false
+    default: ''
+  post:
+    description: 'shell script to run in the post step'
+    required: false
+    default: ''
+runs:
+  using: 'node24'
+  main: 'index.js'
+  post: 'post.js'