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/actions/node12/index.js

@@ -0,0 +1,15 @@
+const core = require('@actions/core');
+const github = require('@actions/github');
+
+try {
+  // `who-to-greet` input defined in action metadata file
+  const nameToGreet = core.getInput('who-to-greet');
+  console.log(`Hello ${nameToGreet}!`);
+  const time = (new Date()).toTimeString();
+  core.setOutput("time", time);
+  // Get the JSON webhook payload for the event that triggered the workflow
+  const payload = JSON.stringify(github.context.payload, undefined, 2)
+  console.log(`The event payload: ${payload}`);
+} catch (error) {
+  core.setFailed(error.message);
+}