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/issue-598/spelling.yml

@@ -0,0 +1,21 @@
+name: issue-598
+on: push
+
+
+jobs:
+  my_first_job:
+
+    runs-on: ubuntu-latest
+    steps:
+      - name: My first false step
+        if: "endsWith('Hello world', 'o1')"
+        run: exit 1
+      - name: My first true step
+        if: "!endsWith('Hello world', 'od')"
+        run: echo "Renst the Octocat"
+      - name: My second false step
+        if: "endsWith('Hello world', 'o2')"
+        run: exit 1
+      - name: My third false step
+        if: "endsWith('Hello world', 'o2')"
+        run: exit 1