Fix lint errors and bump golangci-lint to v2/action v9

- Migrate .golangci.yml to golangci-lint v2 config format
- Bump golangci-lint-action from v6 (Node 20, deprecated) to v9 (Node 24)
- Fix errcheck: check fmt.Fscanln and fmt.Sscanf return values
- Fix ineffassign: replace unused vars with _ in test files
- Fix revive unused-parameter: rename unused cmd/args/r/c params to _
- Fix revive empty-block: invert if condition in push command
- Fix revive redefines-builtin-id: rename 'clear' to 'clearsigned'
- Fix revive blank-imports: add comment on modernc.org/sqlite import
- Fix unused: remove unused 'suite' and 'gen' fields from cache entry

shared/apt/apt.go

@@ -115,11 +115,11 @@ func Generate(s *Suite, signer Signer) (*Indices, error) {
 	idx.Release = release
 
 	if signer != nil {
-		clear, err := signer.ClearSign(release)
+		clearsigned, err := signer.ClearSign(release)
 		if err != nil {
 			return nil, fmt.Errorf("clearsign: %w", err)
 		}
-		idx.InRelease = clear
+		idx.InRelease = clearsigned
 		det, err := signer.DetachedSign(release)
 		if err != nil {
 			return nil, fmt.Errorf("detach sign: %w", err)