Merge gitea/act into act/

Merges the `gitea.com/gitea/act` fork into this repository as the `act/`
directory and consumes it as a local package. The `replace github.com/nektos/act
=> gitea.com/gitea/act` directive is removed; act's dependencies are merged
into the root `go.mod`.

- Imports rewritten: `github.com/nektos/act/pkg/...` → `gitea.com/gitea/act_runner/act/...`
  (flattened — `pkg/` boundary dropped to match the layout forgejo-runner adopted).
- Dropped act's CLI (`cmd/`, `main.go`) and all upstream project files; kept
  the library tree + `LICENSE`.
- Added `// Copyright <year> The Gitea Authors ...` / `// Copyright <year> nektos`
  headers to 104 `.go` files.
- Pre-existing act lint violations annotated inline with
  `//nolint:<linter> // pre-existing issue from nektos/act`.
  `.golangci.yml` is unchanged vs `main`.
- Makefile test target: `-race -short` (matches forgejo-runner).
- Pre-existing integration test failures fixed: race in parallel executor
  (atomic counters); TestSetupEnv / command_test / expression_test /
  run_context_test updated to match gitea fork runtime; TestJobExecutor and
  TestActionCache gated on `testing.Short()`.

Full `gitea/act` commit history is reachable via the second parent.

Co-Authored-By: Claude (Opus 4.7) <noreply@anthropic.com>

act/model/testdata/strategy/push.yml

@@ -0,0 +1,50 @@
+---
+jobs:
+  strategy-all:
+    name: ${{ matrix.node-version }} | ${{ matrix.site }} | ${{ matrix.datacenter }}
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo 'Hello!'
+    strategy:
+      fail-fast: false
+      matrix:
+        datacenter:
+          - site-c
+          - site-d
+        exclude:
+          - datacenter: site-d
+            node-version: 14.x
+            site: staging
+        include:
+          - php-version: 5.4
+          - datacenter: site-a
+            node-version: 10.x
+            site: prod
+          - datacenter: site-b
+            node-version: 12.x
+            site: dev
+        node-version: [14.x, 16.x]
+        site:
+          - staging
+      max-parallel: 2
+  strategy-no-matrix:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo 'Hello!'
+    strategy:
+      fail-fast: false
+      max-parallel: 2
+  strategy-only-fail-fast:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo 'Hello!'
+    strategy:
+      fail-fast: false
+  strategy-only-max-parallel:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo 'Hello!'
+    strategy:
+      max-parallel: 2
+'on':
+  push: null