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/fail/push.yml

@@ -0,0 +1,12 @@
+name: fail
+on: push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: node:24-bookworm-slim
+      env:
+        TEST_ENV: test-value
+    steps:
+      - run: echo ${TEST_ENV} | grep bazooka