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/.gitignore

@@ -0,0 +1,2 @@
+node_modules
+package-lock.json

internal/act/runner/testdata/actions/node24/action.yml

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+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'

internal/act/runner/testdata/actions/node24/index.js

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+import {appendFileSync, readFileSync} from 'node:fs';
+
+const nameToGreet = process.env['INPUT_WHO-TO-GREET'] || 'World';
+console.log(`Hello ${nameToGreet}!`);
+
+if (process.env.GITHUB_OUTPUT) {
+  appendFileSync(process.env.GITHUB_OUTPUT, `time=${new Date().toTimeString()}\n`);
+}
+
+let payload = {};
+if (process.env.GITHUB_EVENT_PATH) {
+  payload = JSON.parse(readFileSync(process.env.GITHUB_EVENT_PATH, 'utf8'));
+}
+console.log(`The event payload: ${JSON.stringify(payload, undefined, 2)}`);

internal/act/runner/testdata/actions/node24/package.json

@@ -0,0 +1,5 @@
+{
+  "name": "node24",
+  "private": true,
+  "type": "module"
+}