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/artifactcache/storage.go

@@ -6,6 +6,7 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"strconv"
 )
 
 type Storage struct {
@@ -103,11 +104,11 @@ func (s *Storage) Remove(id uint64) {
 }
 
 func (s *Storage) filename(id uint64) string {
-	return filepath.Join(s.rootDir, fmt.Sprintf("%02x", id%0xff), fmt.Sprint(id))
+	return filepath.Join(s.rootDir, fmt.Sprintf("%02x", id%0xff), strconv.FormatUint(id, 10))
 }
 
 func (s *Storage) tempDir(id uint64) string {
-	return filepath.Join(s.rootDir, "tmp", fmt.Sprint(id))
+	return filepath.Join(s.rootDir, "tmp", strconv.FormatUint(id, 10))
 }
 
 func (s *Storage) tempName(id uint64, offset int64) string {