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/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