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/res/trampoline.js

@@ -0,0 +1,14 @@
+const { spawnSync } = require('child_process')
+const spawnArguments = {
+  cwd: process.env.INPUT_CWD,
+  stdio: [
+    process.stdin,
+    process.stdout,
+    process.stderr
+  ]
+}
+const child = spawnSync(
+  '/bin/sh',
+  ['-c'].concat(process.env.INPUT_COMMAND),
+  spawnArguments)
+process.exit(child.status)