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/uses-composite-with-error/composite_action2/action.yml

@@ -0,0 +1,11 @@
+---
+name: "Test Composite Action"
+description: "Test action uses composite"
+
+runs:
+  using: "composite"
+  steps:
+  - run: exit 1
+    shell: bash
+  - run: echo should not run in composite steps
+    shell: bash