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/git/git_test.go

@@ -17,7 +17,7 @@ import (
 func TestFindGitSlug(t *testing.T) {
 	assert := assert.New(t)
 
-	var slugTests = []struct {
+	slugTests := []struct {
 		url      string // input
 		provider string // expected result
 		slug     string // expected result
@@ -44,10 +44,7 @@ func TestFindGitSlug(t *testing.T) {
 }
 
 func testDir(t *testing.T) string {
-	basedir, err := os.MkdirTemp("", "act-test")
-	require.NoError(t, err)
-	t.Cleanup(func() { _ = os.RemoveAll(basedir) })
-	return basedir
+	return t.TempDir()
 }
 
 func cleanGitHooks(dir string) error {
@@ -163,8 +160,6 @@ func TestGitFindRef(t *testing.T) {
 			},
 		},
 	} {
-		tt := tt
-		name := name
 		t.Run(name, func(t *testing.T) {
 			dir := filepath.Join(basedir, name)
 			require.NoError(t, os.MkdirAll(dir, 0o755))