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/ensure-post-steps/action-composite/action.yml

@@ -0,0 +1,10 @@
+name: "action composite"
+description: "action composite"
+runs:
+  using: composite
+  steps:
+    # second post action should fail if executed (we do check on the exit code)
+    - uses: ./ensure-post-steps/action-post/
+      with:
+        fail: "true"
+    - uses: ./ensure-post-steps/action-post/

act/runner/testdata/ensure-post-steps/action-post/action.yml

@@ -0,0 +1,11 @@
+name: "action post"
+description: "action post"
+inputs:
+  fail:
+    description: "true if this should fail"
+    required: false
+    default: "false"
+runs:
+  using: node16
+  main: "./main.js"
+  post: "./post.js"

act/runner/testdata/ensure-post-steps/action-post/post.js

@@ -0,0 +1,3 @@
+if (process.env["INPUT_FAIL"] === "true") {
+  process.exit(1);
+}

act/runner/testdata/ensure-post-steps/push.yml

@@ -0,0 +1,8 @@
+name: test
+on: push
+jobs:
+  second-post-step-should-fail:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: ./ensure-post-steps/action-composite/