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/container/testdata/Dockerfile

@@ -0,0 +1,5 @@
+FROM scratch
+ENV PATH="/this/path/does/not/exists/anywhere:/this/either"
+ENV SOME_RANDOM_VAR=""
+ENV ANOTHER_ONE="BUT_I_HAVE_VALUE"
+ENV CONFLICT_VAR="I_EXIST_ONLY_HERE"

act/container/testdata/docker-pull-options/config.json

@@ -0,0 +1,7 @@
+{
+  "auths": {
+          "https://index.docker.io/v1/": {
+                  "auth": "dXNlcm5hbWU6cGFzc3dvcmQK"
+          }
+  }
+}

act/container/testdata/scratch/test.txt

@@ -0,0 +1 @@
+testfile

act/container/testdata/utf8.env

@@ -0,0 +1,3 @@
+FOO=BAR
+HELLO=您好
+BAR=FOO

act/container/testdata/valid.env

@@ -0,0 +1 @@
+ENV1=value1

act/container/testdata/valid.label

@@ -0,0 +1 @@
+LABEL1=value1