Update configuration and environment probing for improved flexibility and compatibility

- Modified the `load_config` function to allow launch preferences in `gpu-rent.vars` to override `.env` settings for non-secret variables, enhancing user control.
- Updated the list of Python candidates in `stack_env_probe.py` to include additional paths for ComfyUI, improving the detection of Python environments.
- Added new pip candidates in `tune_swarm_perf.py` to support various ComfyUI installations, ensuring better compatibility with different setups.

src/gpu_rent/config.py

@@ -143,8 +143,9 @@ def load_config(*, require_auth: bool = True) -> Config:
     if env_file.is_file():
         load_dotenv(env_file, override=False)
 
-    # Non-secret launch defaults (gpu-rent.vars). Do not override .env / real env.
-    apply_vars_file(vars_path(), override=False)
+    # Launch prefs (gpu-rent.vars) win over .env for non-secrets — so
+    # LLM_RUNTIME=ollama in vars is not blocked by LLM_RUNTIME=none from env.example.
+    apply_vars_file(vars_path(), override=True)
 
     missing: list[str] = []
     required = (