Tool for collection discovery

[kacperlukawski]

The server should already be able to use multiple collections after merging #26. However, the model in the MCP client still doesn't know what the options are. There should be an additional tool exposing all the collections available within a cluster so the MCP client can first list them and then select the appropriate one.

[kacperlukawski]

As suggested, we should store collection descriptions in a special collection. The special collection would store technical metadata and for-agent descriptions.