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/mask-values/composite/action.yml

@@ -0,0 +1,12 @@
+name: composite
+description: composite
+
+runs:
+  using: composite
+  steps:
+    - run: echo "secret value"
+      shell: bash
+    - run: echo "::add-mask::$(echo "abc" | base64)"
+      shell: bash
+    - run: echo "abc" | base64
+      shell: bash

internal/act/runner/testdata/mask-values/push.yml

@@ -0,0 +1,12 @@
+name: mask-values
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - run: echo "::add-mask::secret value"
+      - run: echo "secret value"
+      - uses: ./mask-values/composite
+      - run: echo "YWJjCg=="