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

server/restapi/repos_test.go

@@ -31,7 +31,7 @@ func TestRepoCreateAndPermissions(t *testing.T) {
 	}
 
 	// bob cannot see it (private, not a member)
-	code, body = h.do("GET", "/api/v1/repositories/myrepo", bob.Token, nil)
+	code, _ = h.do("GET", "/api/v1/repositories/myrepo", bob.Token, nil)
 	if code != 403 {
 		t.Fatalf("bob should be forbidden from private repo, got %d", code)
 	}