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/prompt_store/src/prompts.rs

@@ -19,20 +19,29 @@ use util::{ResultExt, get_system_shell};
 #[derive(Debug, Clone, Serialize)]
 pub struct ProjectContext {
     pub worktrees: Vec<WorktreeContext>,
+    /// Whether any worktree has a rules_file. Provided as a field because handlebars can't do this.
     pub has_rules: bool,
+    pub default_user_rules: Vec<DefaultUserRulesContext>,
+    /// `!default_user_rules.is_empty()` - provided as a field because handlebars can't do this.
+    pub has_default_user_rules: bool,
     pub os: String,
     pub arch: String,
     pub shell: String,
 }
 
 impl ProjectContext {
-    pub fn new(worktrees: Vec<WorktreeContext>) -> Self {
+    pub fn new(
+        worktrees: Vec<WorktreeContext>,
+        default_user_rules: Vec<DefaultUserRulesContext>,
+    ) -> Self {
         let has_rules = worktrees
             .iter()
             .any(|worktree| worktree.rules_file.is_some());
         Self {
             worktrees,
             has_rules,
+            has_default_user_rules: !default_user_rules.is_empty(),
+            default_user_rules,
             os: std::env::consts::OS.to_string(),
             arch: std::env::consts::ARCH.to_string(),
             shell: get_system_shell(),
@@ -40,6 +49,12 @@ impl ProjectContext {
     }
 }
 
+#[derive(Debug, Clone, Serialize)]
+pub struct DefaultUserRulesContext {
+    pub title: Option<String>,
+    pub contents: String,
+}
+
 #[derive(Debug, Clone, Serialize)]
 pub struct WorktreeContext {
     pub root_name: String,
@@ -377,3 +392,30 @@ impl PromptBuilder {
         self.handlebars.lock().render("suggest_edits", &())
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_assistant_system_prompt_renders() {
+        let worktrees = vec![WorktreeContext {
+            root_name: "path".into(),
+            abs_path: Path::new("/some/path").into(),
+            rules_file: Some(RulesFileContext {
+                path_in_worktree: Path::new(".rules").into(),
+                abs_path: Path::new("/some/path/.rules").into(),
+                text: "".into(),
+            }),
+        }];
+        let default_user_rules = vec![DefaultUserRulesContext {
+            title: Some("Rules title".into()),
+            contents: "Rules contents".into(),
+        }];
+        let project_context = ProjectContext::new(worktrees, default_user_rules);
+        PromptBuilder::new(None)
+            .unwrap()
+            .generate_assistant_system_prompt(&project_context)
+            .unwrap();
+    }
+}