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/job-needs-context-contains-result/push.yml

@@ -0,0 +1,15 @@
+on:
+  push:
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - run: exit 0
+  assert:
+    needs: test
+    if: |
+      ( always() && !cancelled() ) && (
+      ( needs.test.result != 'success' || !success() ) )
+    runs-on: ubuntu-latest
+    steps:
+    - run: exit 1