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/assistant_panel.rs

@@ -213,7 +213,7 @@ impl AssistantPanel {
                     let project = workspace.project().clone();
                     ThreadStore::load(project, tools.clone(), prompt_builder.clone(), cx)
                 })?
-                .await;
+                .await?;
 
             let slash_commands = Arc::new(SlashCommandWorkingSet::default());
             let context_store = workspace