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/pull-request/event.json

@@ -0,0 +1,10 @@
+{
+  "pull_request": {
+    "head": {
+      "ref": "sample-head-ref"
+    },
+    "base": {
+      "ref": "sample-base-ref"
+    }
+  }
+}

internal/act/runner/testdata/pull-request/main.yaml

@@ -0,0 +1,26 @@
+name: basic
+on: pull_request
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      # test refs from event.json
+      - run: echo '${{github.ref}}'
+      - run: echo '${{github.head_ref}}' | grep sample-head-ref
+      - run: echo '${{github.base_ref}}' | grep sample-base-ref
+      # test main/composite context equality with data from event.json
+      - run: |
+          runs:
+            using: composite
+            steps:
+            - run: |
+                echo WORKFLOW_GITHUB_CONTEXT="$WORKFLOW_GITHUB_CONTEXT"
+                echo COMPOSITE_GITHUB_CONTEXT="$COMPOSITE_GITHUB_CONTEXT"
+                [[ "$WORKFLOW_GITHUB_CONTEXT" = "$COMPOSITE_GITHUB_CONTEXT" ]]
+              env:
+                WORKFLOW_GITHUB_CONTEXT: ${{ tojson(tojson(github.event)) }}
+                COMPOSITE_GITHUB_CONTEXT: ${{ '${{tojson(github.event)}}' }}
+              shell: bash
+        shell: cp {0} action.yml
+      - uses: ./