Add "inactivity logout" caveat

This would be a caveat for RPC That allows a given permission to be limited to a maximum time. This can be useful for any permission that needs to be limited for an amount of time, but could be a pretty simple caveat to add that would allow for a feature like "log me out after X minutes" or "log me out after X weeks".

This would be a feature addition, and at some point I'm sure we would want to allow users to select a limit to the duration of a data connection, but it isn't a pressing necessity for now.

The issue does not name any files, tests, or entry points. Begin by tracing the RPC caveat and permission-duration handling, then clarify the intended inactivity/logout behavior and completion criteria before implementation.

Written by the indexing model from the issue text.