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/matrix-exitcode/push.yml

@@ -0,0 +1,16 @@
+name: test
+
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        val: ["success", "failure"]
+      fail-fast: false
+    steps:
+      - name: test
+        run: |
+          echo "Expected job result: ${{ matrix.val }}"
+          [[ "${{ matrix.val }}" = "success" ]] || exit 1