agent: Support adding selection as context (#28964)

https://github.com/user-attachments/assets/42ebe911-3392-48f7-8583-caab285aca09

Release Notes:

- agent: Support adding selections via @selection or `assistant: Quote
selection` as context
002235d0daBennet Bo Fenner committed on 4/17/2025, 2:55:15 PM· committed by GitHubparentf07695c
9 files changedLine totals unavailable