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/common/cartesian_test.go

@@ -0,0 +1,43 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// Copyright 2020 The nektos/act Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCartesianProduct(t *testing.T) {
+	assert := assert.New(t)
+	input := map[string][]any{
+		"foo": {1, 2, 3, 4},
+		"bar": {"a", "b", "c"},
+		"baz": {false, true},
+	}
+
+	output := CartesianProduct(input)
+	assert.Len(output, 24)
+
+	for _, v := range output {
+		assert.Len(v, 3)
+
+		assert.Contains(v, "foo")
+		assert.Contains(v, "bar")
+		assert.Contains(v, "baz")
+	}
+
+	input = map[string][]any{
+		"foo": {1, 2, 3, 4},
+		"bar": {},
+		"baz": {false, true},
+	}
+	output = CartesianProduct(input)
+	assert.Len(output, 0) //nolint:testifylint // pre-existing issue from nektos/act
+
+	input = map[string][]any{}
+	output = CartesianProduct(input)
+	assert.Len(output, 0) //nolint:testifylint // pre-existing issue from nektos/act
+}