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/agent/src/agent_diff.rs

@@ -922,6 +922,7 @@ mod tests {
             language::init(cx);
             Project::init_settings(cx);
             AssistantSettings::register(cx);
+            prompt_store::init(cx);
             thread_store::init(cx);
             workspace::init_settings(cx);
             ThemeSettings::register(cx);
@@ -951,7 +952,8 @@ mod tests {
                     cx,
                 )
             })
-            .await;
+            .await
+            .unwrap();
         let thread = thread_store.update(cx, |store, cx| store.create_thread(cx));
         let action_log = thread.read_with(cx, |thread, _| thread.action_log().clone());