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/ensure-post-steps/action-composite/action.yml

@@ -0,0 +1,10 @@
+name: "action composite"
+description: "action composite"
+runs:
+  using: composite
+  steps:
+    # second post action should fail if executed (we do check on the exit code)
+    - uses: ./ensure-post-steps/action-post/
+      with:
+        fail: "true"
+    - uses: ./ensure-post-steps/action-post/