Use golangci-lint fmt to format code (#163)

Use `golangci-lint fmt` to format code, upgrading `.golangci.yml` to v2 and mirroring the linter configuration used by https://github.com/go-gitea/gitea. `gci` now handles import ordering into standard, project-local, blank, and default groups.

Mirrors https://github.com/go-gitea/gitea/pull/37194.

Changes:
- Upgrade `.golangci.yml` to v2 format with the same linter set as gitea (minus `prealloc`, `unparam`, `testifylint`, `nilnil` which produced too many pre-existing issues)
- Add path-based exclusions (`bodyclose`, `gosec` in tests; `gosec:G115`/`G117` globally)
- Run lint via `make lint-go` in CI instead of `golangci/golangci-lint-action`, matching the pattern used by other Gitea repos
- Apply safe auto-fixes (`modernize`, `perfsprint`, `usetesting`, etc.)
- Add explanations to existing `//nolint` directives
- Remove dead code (unused `newRemoteReusableWorkflow` and `networkName`), duplicate imports, and shadowed `max` builtins
- Replace deprecated `docker/distribution/reference` with `distribution/reference`
- Fix `Deprecated:` comment casing and simplify nil/len checks

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/act/pulls/163
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

pkg/common/executor_test.go

@@ -2,7 +2,7 @@ package common
 
 import (
 	"context"
-	"fmt"
+	"errors"
 	"testing"
 	"time"
 
@@ -19,7 +19,7 @@ func TestNewWorkflow(t *testing.T) {
 	assert.Nil(emptyWorkflow(ctx))
 
 	// error case
-	errorWorkflow := NewErrorExecutor(fmt.Errorf("test error"))
+	errorWorkflow := NewErrorExecutor(errors.New("test error"))
 	assert.NotNil(errorWorkflow(ctx))
 
 	// multiple success case
@@ -122,7 +122,7 @@ func TestNewParallelExecutorFailed(t *testing.T) {
 	count := 0
 	errorWorkflow := NewPipelineExecutor(func(ctx context.Context) error {
 		count++
-		return fmt.Errorf("fake error")
+		return errors.New("fake error")
 	})
 	err := NewParallelExecutor(1, errorWorkflow)(ctx)
 	assert.Equal(1, count)
@@ -135,7 +135,7 @@ func TestNewParallelExecutorCanceled(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
-	errExpected := fmt.Errorf("fake error")
+	errExpected := errors.New("fake error")
 
 	count := 0
 	successWorkflow := NewPipelineExecutor(func(ctx context.Context) error {