Rename assistant2 to agent (#27887)

This PR renames the `assistant2` crate to `agent`.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -16,10 +16,10 @@ path = "src/main.rs"
 
 [dependencies]
 activity_indicator.workspace = true
+agent.workspace = true
 anyhow.workspace = true
 assets.workspace = true
 assistant.workspace = true
-assistant2.workspace = true
 assistant_context_editor.workspace = true
 assistant_settings.workspace = true
 assistant_tools.workspace = true