Call them coworkers, everywhere (#11)

One word for the thing, used consistently: the README, the docs, the UI copy,
the comments and the tests. Renames docs/teammates.md and the schema module
that matched it, and regenerates the architecture diagram, whose server box
carried the old word too.

No database object changes. The tables were already agent_profiles and
agent_preferences, so the schema file was only ever a module name.