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-status-check/push.yml

@@ -0,0 +1,28 @@
+on: push
+jobs:
+  fail:
+    runs-on: ubuntu-latest
+    steps:
+    - run: exit 1
+  suc1:
+    if: success() || failure()
+    needs:
+    - fail
+    runs-on: ubuntu-latest
+    steps:
+    - run: exit 0
+  suc2:
+    if: success() || failure()
+    needs:
+    - fail
+    runs-on: ubuntu-latest
+    steps:
+    - run: exit 0
+  next:
+    needs:
+    - suc1
+    - suc2
+    runs-on: ubuntu-latest
+    steps:
+    - run: echo should never reach here
+    - run: exit 1