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/act-composite-env-test/action2/action.yml

@@ -0,0 +1,21 @@
+name: action2
+description: actions2
+runs:
+  using: composite
+  steps:
+  - name: env.COMPOSITE_OVERRIDE != '2'
+    run: exit 1
+    if: env.COMPOSITE_OVERRIDE != '2'
+    shell: bash
+  - name: env.COMPOSITE != '1'
+    run: exit 1
+    if: env.COMPOSITE != '1'
+    shell: bash
+  - name: env.JOB != '1'
+    run: exit 1
+    if: env.JOB != '1'
+    shell: bash
+  - name: env.GLOBAL != '1'
+    run: exit 1
+    if: env.GLOBAL != '1'
+    shell: bash