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/actions/node24/action.yml

@@ -0,0 +1,13 @@
+name: 'Hello World'
+description: 'Greet someone and record the time'
+inputs:
+  who-to-greet:  # id of input
+    description: 'Who to greet'
+    required: true
+    default: 'World'
+outputs:
+  time: # id of output
+    description: 'The time we greeted you'
+runs:
+  using: 'node24'
+  main: 'index.js'