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/issue-1595/no-event.yml

@@ -0,0 +1,8 @@
+name: no event
+
+jobs:
+  stuck:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo How did you get here?
+        shell: bash