agent: Use default prompts from prompt library in system prompt (#28915)

Related to #28490.

- Default prompts from the prompt library are now included as "user
rules" in the system prompt.
- Presence of these user rules is shown at the beginning of the thread
in the UI.
_ Now uses an `Entity<PromptStore>` instead of an `Arc<PromptStore>`.
Motivation for this is emitting a `PromptsUpdatedEvent`.
- Now disallows concurrent reloading of the system prompt. Before this
change it was possible for reloads to race.

Release Notes:

- agent: Added support for including default prompts from the Prompt
Library as "user rules" in the system prompt.

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/eval/src/example.rs

@@ -309,7 +309,7 @@ impl Example {
                 return Err(anyhow!("Setup only mode"));
             }
 
-            let thread_store = thread_store.await;
+            let thread_store = thread_store.await?;
             let thread =
                 thread_store.update(cx, |thread_store, cx| thread_store.create_thread(cx))?;