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.go

@@ -0,0 +1,58 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// Copyright 2020 The nektos/act Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+// CartesianProduct takes map of lists and returns list of unique tuples
+func CartesianProduct(mapOfLists map[string][]any) []map[string]any {
+	listNames := make([]string, 0)
+	lists := make([][]any, 0)
+	for k, v := range mapOfLists {
+		listNames = append(listNames, k)
+		lists = append(lists, v)
+	}
+
+	listCart := cartN(lists...)
+
+	rtn := make([]map[string]any, 0)
+	for _, list := range listCart {
+		vMap := make(map[string]any)
+		for i, v := range list {
+			vMap[listNames[i]] = v
+		}
+		rtn = append(rtn, vMap)
+	}
+	return rtn
+}
+
+func cartN(a ...[]any) [][]any {
+	c := 1
+	for _, a := range a {
+		c *= len(a)
+	}
+	if c == 0 || len(a) == 0 {
+		return nil
+	}
+	p := make([][]any, c)
+	b := make([]any, c*len(a))
+	n := make([]int, len(a))
+	s := 0
+	for i := range p {
+		e := s + len(a)
+		pi := b[s:e]
+		p[i] = pi
+		s = e
+		for j, n := range n {
+			pi[j] = a[j][n]
+		}
+		for j := len(n) - 1; j >= 0; j-- {
+			n[j]++
+			if n[j] < len(a[j]) {
+				break
+			}
+			n[j] = 0
+		}
+	}
+	return p
+}