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/filecollector/file_collector.go

@@ -3,6 +3,7 @@ package filecollector
 import (
 	"archive/tar"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"io/fs"
@@ -97,8 +98,7 @@ type Fs interface {
 	Readlink(path string) (string, error)
 }
 
-type DefaultFs struct {
-}
+type DefaultFs struct{}
 
 func (*DefaultFs) Walk(root string, fn filepath.WalkFunc) error {
 	return filepath.Walk(root, fn)
@@ -124,7 +124,7 @@ func (*DefaultFs) Readlink(path string) (string, error) {
 	return os.Readlink(path)
 }
 
-//nolint:gocyclo
+//nolint:gocyclo // function handles many cases
 func (fc *FileCollector) CollectFiles(ctx context.Context, submodulePath []string) filepath.WalkFunc {
 	i, _ := fc.Fs.OpenGitIndex(path.Join(fc.SrcPath, path.Join(submodulePath...)))
 	return func(file string, fi os.FileInfo, err error) error {
@@ -134,7 +134,7 @@ func (fc *FileCollector) CollectFiles(ctx context.Context, submodulePath []strin
 		if ctx != nil {
 			select {
 			case <-ctx.Done():
-				return fmt.Errorf("copy cancelled")
+				return errors.New("copy cancelled")
 			default:
 			}
 		}