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/inputs-via-env-context/action.yml

@@ -0,0 +1,8 @@
+inputs:
+  test-env-input: {}
+runs:
+  using: composite
+  steps:
+  - run: |
+      exit ${{ inputs.test-env-input == env.test-env-input && '0' || '1'}}
+    shell: bash