Standardize usage of username and fullname / email throughout

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We reference email, username, and full name throughout but we're not consistent about where they are used and when.

Let's come up with a simple guide line for when we use each one and how we refer to them. For instance, in the path we refer to `identity`, but this is a username or machine name. Other places, we use the full name and username.

If we could standardize, it'd be easier for a user to grok the system.

The issue does not identify files, tests, or entry points. First inventory how email, username, full name, and identity are currently named and used across the project, then agree on terminology and document the resulting guidance. Done means the usage rules are explicit and consistently applied throughout the system.

Written by the indexing model from the issue text.