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/path-handling/push.yml

@@ -0,0 +1,39 @@
+name: path tests
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: "Append to $GITHUB_PATH"
+        run: |
+          echo "/opt/hostedtoolcache/node/18.99/x64/bin" >> $GITHUB_PATH
+ 
+      - name: test path (after setup)
+        run: |
+          if ! echo "$PATH" |grep "/opt/hostedtoolcache/node/18.*/\(x64\|arm64\)/bin" ; then
+            echo "Node binaries not in path: $PATH"
+            exit 1
+          fi
+
+      - id: action-with-output
+        uses: ./path-handling/
+
+      - name: test path (after local action)
+        run: |
+          if ! echo "$PATH" |grep "/opt/hostedtoolcache/node/18.*/\(x64\|arm64\)/bin" ; then
+            echo "Node binaries not in path: $PATH"
+            exit 1
+          fi
+
+      - uses: ./path-handling/
+        with:
+          input: some input
+
+      - name: test path (after remote action)
+        run: |
+          if ! echo "$PATH" |grep "/opt/hostedtoolcache/node/18.*/\(x64\|arm64\)/bin" ; then
+            echo "Node binaries not in path: $PATH"
+            exit 1
+          fi