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-environment-and-context-tests/docker/Dockerfile

@@ -0,0 +1,5 @@
+FROM alpine:3.24
+
+COPY entrypoint.sh /entrypoint.sh
+
+ENTRYPOINT [ "/entrypoint.sh" ]

internal/act/runner/testdata/actions-environment-and-context-tests/docker/action.yml

@@ -0,0 +1,5 @@
+name: 'Test'
+description: 'Test'
+runs:
+  using: 'docker'
+  image: 'Dockerfile'

internal/act/runner/testdata/actions-environment-and-context-tests/docker/entrypoint.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+checkEnvVar() {
+	name=$1
+	value=$2
+	allowEmpty=$3
+
+	if [ -z "$value" ]; then
+		echo "Missing environment variable: $name"
+		exit 1
+	fi
+
+	if [ "$allowEmpty" != "true" ]; then
+		test=$(echo "$value" |cut -f 2 -d "=")
+		if [ -z "$test" ]; then
+			echo "Environment variable is empty: $name"
+			exit 1
+		fi
+	fi
+
+	echo "$value"
+}
+
+checkEnvVar "GITHUB_ACTION" "$(env |grep "GITHUB_ACTION=")" false
+checkEnvVar "GITHUB_ACTION_REPOSITORY" "$(env |grep "GITHUB_ACTION_REPOSITORY=")" true
+checkEnvVar "GITHUB_ACTION_REF" "$(env |grep "GITHUB_ACTION_REF=")" true