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/inputs-via-env-context/push.yml

@@ -0,0 +1,15 @@
+on: push
+jobs:
+  test-inputs-via-env-context:
+    runs-on: self-hosted
+    steps:
+    - uses: actions/checkout@v3
+    - uses: ./inputs-via-env-context
+      with:
+        test-env-input: ${{ env.test-env-input }}
+      env:
+        test-env-input: ${{ github.event_name }}/${{ github.run_id }}
+    - run: |
+        exit ${{ env.test-env-input == format('{0}/{1}', github.event_name, github.run_id) && '0' || '1' }}
+      env:
+        test-env-input: ${{ github.event_name }}/${{ github.run_id }}