The TAG has published the @w3c Group Note: User Agents
https://www.w3.org/TR/user-agents/
It describes what makes software a user agent, e.g., browsers, search engines, voice-driven assistants, and generative AI systems acting on a person's behalf, and the duties owed to the user: protection, honesty, and loyalty.
Edited by @jyasskin and @csarven . Thanks to everyone who contributed!
Feedback welcome: https://github.com/w3ctag/user-agents/