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/shells/sh/push.yml

@@ -0,0 +1,37 @@
+on: push
+env:
+  MY_SHELL: sh
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - shell: ${{ env.MY_SHELL }}
+        run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
+          fi
+  check-container:
+    runs-on: ubuntu-latest
+    container: alpine:latest
+    steps:
+      - shell: ${{ env.MY_SHELL }}
+        run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
+          fi
+  check-job-default:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: ${{ env.MY_SHELL }}
+    steps:
+      - run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
+          fi