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/ensure-post-steps/action-post/action.yml

@@ -0,0 +1,11 @@
+name: "action post"
+description: "action post"
+inputs:
+  fail:
+    description: "true if this should fail"
+    required: false
+    default: "false"
+runs:
+  using: node24
+  main: "./main.js"
+  post: "./post.js"

internal/act/runner/testdata/ensure-post-steps/action-post/post.js

@@ -0,0 +1,3 @@
+if (process.env["INPUT_FAIL"] === "true") {
+  process.exit(1);
+}