Introduce a system prompt for the new assistant (#26536)

This should be less eager in terms of invoking tools. But we should keep
iterating on it as we add more tools.

Also, this disables the Lua interpreter by default (it can still be
enabled manually from the tools icon).

Release Notes:

- N/A

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/assistant2/src/system_prompt.md

@@ -0,0 +1,12 @@
+You are an AI assistant integrated into a text editor. Your goal is to do one of the following two things:
+
+1. Help users answer questions and perform tasks related to their codebase.
+2. Answer general-purpose questions unrelated to their particular codebase.
+
+It will be up to you to decide which of these you are doing based on what the user has told you. When unclear, ask clarifying questions to understand the user's intent before proceeding.
+
+You should only perform actions that modify the user’s system if explicitly requested by the user:
+- If the user asks a question about how to accomplish a task, provide guidance or information, and use read-only tools (e.g., search) to assist. You may suggest potential actions, but do not directly modify the user’s system without explicit instruction.
+- If the user clearly requests that you perform an action, carry out the action directly without explaining why you are doing so.
+
+Be concise and direct in your responses.

crates/assistant2/src/thread.rs

@@ -376,7 +376,13 @@ impl Thread {
         _cx: &App,
     ) -> LanguageModelRequest {
         let mut request = LanguageModelRequest {
-            messages: vec![],
+            messages: vec![LanguageModelRequestMessage {
+                role: Role::System,
+                content: vec![MessageContent::Text(
+                    include_str!("./system_prompt.md").to_string(),
+                )],
+                cache: true,
+            }],
             tools: Vec::new(),
             stop: Vec::new(),
             temperature: None,

crates/assistant_tool/src/tool_working_set.rs

@@ -15,7 +15,6 @@ pub struct ToolWorkingSet {
     state: Mutex<WorkingSetState>,
 }
 
-#[derive(Default)]
 struct WorkingSetState {
     context_server_tools_by_id: HashMap<ToolId, Arc<dyn Tool>>,
     context_server_tools_by_name: HashMap<String, Arc<dyn Tool>>,
@@ -24,6 +23,18 @@ struct WorkingSetState {
     next_tool_id: ToolId,
 }
 
+impl Default for WorkingSetState {
+    fn default() -> Self {
+        Self {
+            context_server_tools_by_id: Default::default(),
+            context_server_tools_by_name: Default::default(),
+            disabled_tools_by_source: Default::default(),
+            is_scripting_tool_disabled: true,
+            next_tool_id: Default::default(),
+        }
+    }
+}
+
 impl ToolWorkingSet {
     pub fn tool(&self, name: &str, cx: &App) -> Option<Arc<dyn Tool>> {
         self.state