fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/channelserver/handlers_mercenary.go

@@ -325,7 +325,7 @@ func handleMsgMhfSaveOtomoAirou(s *Session, p mhfpacket.MHFPacket) {
 		}
 	}
 	save.WriteBytes(bf.DataFromCurrent())
-	save.Seek(0, 0)
+	_, _ = save.Seek(0, 0)
 	save.WriteUint8(catsExist)
 	comp, err := nullcomp.Compress(save.Data())
 	if err != nil {