* fix(subagents): use model override for tools and middleware
* fix(config): resolve effective subagent model
* fix(subagents): defer app config loading
* fix(subagents): fully defer config.yaml load in executor __init__
The previous attempt only relocated the explicit get_app_config() call,
but left resolve_subagent_model_name(...) running eagerly in __init__.
That helper has its own internal get_app_config() fallback, which still
fired when both app_config and parent_model were None and
config.model == "inherit" — exactly the path unit tests hit, breaking
21 tests in CI with FileNotFoundError: config.yaml.
Skip the eager resolve in __init__ when it would require loading the
config file, and defer to _create_agent (which already has the
app_config or get_app_config() fallback).
487c1d939f