Add support to Zed Agent

[dlight]

The Zed editor has an agent, it would be rad if git-ai supported it out of box. I don't know, however, if Zed currently offers any API to make this possible, like hooks and whatnot.

I previously created a discussion about Zed itself adding support to save AI threads in a repo, zed-industries/zed#45334 (but it didn't gain any traction). But I think something like git-ai receiving threads from Zed would work better.