I'm craving something similar, but a smidge more than tab-completion. I'd like to treat the agent as an ephemeral pair programmer. Give the editor some keybind to mark and comment places in the code, and then I edit a prompt telling it what focused changes to make where I've marked. After a round of changes is good and complete, the next batch starts with a fresh context and no memory of the previous one.

Aider in watching mode is almost this, but I'd have to insert and save actual "AI:" comments for it to replace. It wouldn't surprise me if VS Code already has something like I'm thinking, but VS Code is painful to me in so many other ways I refuse to use it.

I should probably just vibe code a vim or emacs extension for exactly what I want, but I haven't yet.

Ctrl+I kind of does this: https://code.visualstudio.com/docs/chat/inline-chat