Reapply "Use Project instead of Workspace in ContextStore (#28402)" (#28441)

Motivation for this change is to use `ContextStore` in headless
assistant, which requires it to not depend on UI entities like
`Workspace`.

This reapplies a change that was revert was in #28428, and fixes the panic.

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(workspace.downgrade(), None));
+        let context_store = cx.new(|_cx| ContextStore::new(project.downgrade(), None));
 
         (
             workspace,