Rename assistant_settings to agent_settings (#31513)

This PR renames the `assistant_settings` crate to `agent_settings`, as
well a number of constructs within it.

Release Notes:

- N/A

crates/collab/Cargo.toml

@@ -76,8 +76,8 @@ workspace-hack.workspace = true
 zed_llm_client.workspace = true
 
 [dev-dependencies]
+agent_settings.workspace = true
 assistant_context_editor.workspace = true
-assistant_settings.workspace = true
 assistant_slash_command.workspace = true
 assistant_tool.workspace = true
 async-trait.workspace = true