Rename "Prompt Library" to "Rules Library" (#29349)

There's probably more to do to fully make the transition, and we'll
still debate a bit internally whether this is the name, but just opening
this PR up now for visibility.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -101,7 +101,7 @@ pub fn init(
     SlashCommandSettings::register(cx);
 
     assistant_context_editor::init(client.clone(), cx);
-    prompt_library::init(cx);
+    rules_library::init(cx);
     init_language_model_settings(cx);
     assistant_slash_command::init(cx);
     assistant_tool::init(cx);