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/runner/testdata/windows-add-env/action.yml

@@ -0,0 +1,7 @@
+runs:
+  using: composite
+  steps:
+  - run: |
+      echo $env:GITHUB_ENV
+      echo "kEy=n/a" > $env:GITHUB_ENV
+    shell: pwsh

act/runner/testdata/windows-add-env/push.yml

@@ -0,0 +1,41 @@
+on:
+  push:
+jobs:
+  test:
+    runs-on: windows-latest
+    steps:
+    - run: |
+        echo $env:GITHUB_ENV
+        echo "key=val" > $env:GITHUB_ENV
+        echo "key2<<EOF" >> $env:GITHUB_ENV
+        echo "line1" >> $env:GITHUB_ENV
+        echo "line2" >> $env:GITHUB_ENV
+        echo "EOF" >> $env:GITHUB_ENV
+        cat $env:GITHUB_ENV
+    - run: |
+        ls env:
+        if($env:key -ne 'val') {
+          echo "Unexpected value for `$env:key: $env:key"
+          exit 1
+        }
+        if($env:key2 -ne "line1`nline2") {
+          echo "Unexpected value for `$env:key2: $env:key2"
+          exit 1
+        }
+    - run: |
+        echo $env:GITHUB_ENV
+        echo "KEY=test" > $env:GITHUB_ENV
+        echo "Key=expected" > $env:GITHUB_ENV
+    - name: Assert GITHUB_ENV is merged case insensitive
+      run: exit 1
+      if: env.KEY != 'expected' || env.Key != 'expected' || env.key != 'expected'
+    - name: Assert step env is merged case insensitive
+      run: exit 1
+      if: env.KEY != 'n/a' || env.Key != 'n/a' || env.key != 'n/a'
+      env:
+        KeY: 'n/a'
+    - uses: actions/checkout@v3
+    - uses: ./windows-add-env
+    - name: Assert composite env is merged case insensitive
+      run: exit 1
+      if: env.KEY != 'n/a' || env.Key != 'n/a' || env.key != 'n/a'