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/artifactcache/testdata/example/example.yaml

@@ -0,0 +1,30 @@
+# Copied from https://github.com/actions/cache#example-cache-workflow
+name: Caching Primes
+
+on: push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - run: env
+
+      - uses: actions/checkout@v3
+
+      - name: Cache Primes
+        id: cache-primes
+        uses: actions/cache@v3
+        with:
+          path: prime-numbers
+          key: ${{ runner.os }}-primes-${{ github.run_id }}
+          restore-keys: |
+            ${{ runner.os }}-primes
+            ${{ runner.os }}
+
+      - name: Generate Prime Numbers
+        if: steps.cache-primes.outputs.cache-hit != 'true'
+        run: cat /proc/sys/kernel/random/uuid > prime-numbers
+
+      - name: Use Prime Numbers
+        run: cat prime-numbers