chore: Extract PromptStore out of prompt_library (#25837)

One step closer to removing long pole with assistant/assistant2 builds

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -19,7 +19,7 @@ use gpui::{actions, App, Global, UpdateGlobal};
 use language_model::{
     LanguageModelId, LanguageModelProviderId, LanguageModelRegistry, LanguageModelResponseMessage,
 };
-use prompt_library::PromptBuilder;
+use prompt_store::PromptBuilder;
 use semantic_index::{CloudEmbeddingProvider, SemanticDb};
 use serde::Deserialize;
 use settings::{Settings, SettingsStore};