Revert to fix panic in inline assistant (#28428)

This reverts commit f12a554f86, which
introduced a panic in inline assistant (cc @mgsloan) - I'm not sure what
the motivation was for that change, but I figure we can revert to fix
the inline assistant now and deal with that later. 😄

Panic was:

> Thread "main" panicked with "cannot read workspace::Workspace while it
is already being updated" at
/Users/rtfeldman/code/zed/crates/gpui/src/app/entity_map.rs:139:32


Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -2266,7 +2266,7 @@ fn main() {{
         });
 
         let thread = thread_store.update(cx, |store, cx| store.create_thread(cx));
-        let context_store = cx.new(|_cx| ContextStore::new(project.downgrade(), None));
+        let context_store = cx.new(|_cx| ContextStore::new(workspace.downgrade(), None));
 
         (
             workspace,