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/uses-action-with-pre-and-post-step/last-action/action.yml

@@ -0,0 +1,7 @@
+name: "last action check"
+description: "last action check"
+
+runs:
+  using: "node16"
+  main: main.js
+  post: post.js

act/runner/testdata/uses-action-with-pre-and-post-step/last-action/post.js

@@ -0,0 +1,17 @@
+const pre = process.env['ACTION_OUTPUT_PRE'];
+const main = process.env['ACTION_OUTPUT_MAIN'];
+const post = process.env['ACTION_OUTPUT_POST'];
+
+console.log({pre, main, post});
+
+if (pre !== 'pre') {
+  throw new Error(`Expected 'pre' but got '${pre}'`);
+}
+
+if (main !== 'main') {
+  throw new Error(`Expected 'main' but got '${main}'`);
+}
+
+if (post !== 'post') {
+  throw new Error(`Expected 'post' but got '${post}'`);
+}