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/secrets/.secrets

@@ -0,0 +1,3 @@
+MY_SECRET=top-secret
+MULTILINE_SECRET="foo\nbar\nbaz"
+JSON_SECRET={"foo": "bar"}