Anders has answered the question very well. As Anders has already said, Domino, unfortunately, cannot share code libraries between databases. All the code is self contained, which is in this scenario a limitation.
Copying the agent into all the databases you want to use it for, and then employing design inheritance is a quick and easy way to distribute the agent.
An alternative idea, is to have a single database that serves as a repository of agents, so if you need to re-use the same agent over and over, it's design is always in one database, but you will need to design it so that it can perform operations on all the database you need to update.
Effectively, using each database as a datasource only, and the relevant agent(s) operating from one location. It will require some more work that will allow you to define some additional configuration documents that the agent(s) would use to identify which databases you want the agent to run on.
The advantages of this approach are :
You don't need to contend with design inheritance. It can get messy on a large scale when you have complex script library/design structures, you may have to buy third party tools to help you do this.
You can actually control which databases get updated via a series of configuration documents centrally with an "active/inactive" field that flags the database for update. Rather than directly "touching" the agents which requires you to get your hands dirty with enabling/disabling the agent. In some corporate environments which are tightly controlled, you need to keep asking the Notes admin to do this for you.
You can code the agent so that it reports activity in your own custom log documents when it runs on each database, and centrally store it.
Hope this provides you with some options...
