Support calls to Continue tools

The "Plan" and "Agent" modes of the Continue client provide a number of tools for listing files and reading open files, among other things. For task 1.3, we need to expose these tools to the reference agent so it can decide to use some of them in preparation for a software engineering task that should be given to a MOSAICO agent.

We may need an A2A extension to represent this information in the request as well, which should be appropriately documented (e.g. by adding an A2A-EXTENSIONS.md document in the repo?).