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_slash_commands/Cargo.toml

@@ -32,7 +32,7 @@ language.workspace = true
 language_model.workspace = true
 log.workspace = true
 project.workspace = true
-prompt_library.workspace = true
+prompt_store.workspace = true
 rope.workspace = true
 schemars.workspace = true
 semantic_index.workspace = true