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/commands/push.yml

@@ -0,0 +1,42 @@
+name: basic
+on: push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      ACTIONS_ALLOW_UNSECURE_COMMANDS: 'true'
+    steps:
+      - name: TEST set-env
+        run: echo "::set-env name=foo::bar"
+      - name: TEST set-env (cont.)
+        run: echo $foo | grep bar
+
+      - name: TEST set-output
+        id: set_output
+        run: echo "::set-output name=zoo::zar"
+
+      - run: echo "::add-path::/zip"
+      - run: echo $PATH | grep /zip
+
+      - name: TEST conditional
+        if: steps.set_output.outputs.zoo
+        run: echo "::set-env name=cond_env::foo"
+      - name: TEST conditional (cont.)
+        run: echo $cond_env | grep foo
+
+      - name: TEST debug, warning, error
+        run: |
+          echo "::debug file=app.js,line=100,col=20::Hello debug!"
+          echo "::warning file=app.js,line=100,col=20::Hello warning!"
+          echo "::error file=app.js,line=100,col=30::Hello error!"
+
+      - name: TEST stop-commands
+        run: |
+          echo "::stop-commands::my-end-token" 
+          echo "::set-env name=foo::baz"
+          echo $foo | grep bar
+          echo "::my-end-token::"
+          echo "::set-env name=foo::baz"
+      - name: TEST stop-commands (cont.)
+        run: echo $foo | grep baz