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/windows-prepend-path/push.yml

@@ -0,0 +1,34 @@
+on:
+  push:
+defaults:
+  run:
+    shell: pwsh
+jobs:
+  test:
+    runs-on: windows-latest
+    steps:
+    - run: |
+        mkdir build
+        echo '@echo off' > build/test.cmd
+        echo 'echo Hi' >> build/test.cmd
+        mkdir build2
+        echo '@echo off' > build2/test2.cmd
+        echo 'echo test2' >> build2/test2.cmd
+    - run: |
+        echo '${{ tojson(runner) }}'
+        ls
+        echo '${{ github.workspace }}'
+      working-directory: ${{ github.workspace }}\build
+    - run: |
+        echo $env:GITHUB_PATH
+        echo '${{ github.workspace }}\build' > $env:GITHUB_PATH
+        cat $env:GITHUB_PATH
+    - run: |
+        echo $env:PATH
+        test
+    - run: |
+        echo "PATH=$env:PATH;${{ github.workspace }}\build2" > $env:GITHUB_ENV
+    - run: |
+        echo $env:PATH
+        test
+        test2