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/assistant_slash_commands/src/default_command.rs

@@ -54,9 +54,9 @@ impl SlashCommand for DefaultSlashCommand {
         cx: &mut App,
     ) -> Task<SlashCommandResult> {
         let store = PromptStore::global(cx);
-        cx.background_spawn(async move {
+        cx.spawn(async move |cx| {
             let store = store.await?;
-            let prompts = store.default_prompt_metadata();
+            let prompts = store.read_with(cx, |store, _cx| store.default_prompt_metadata())?;
 
             let mut text = String::new();
             text.push('\n');