refactor: make config.yaml the single source of truth for endpoint URLs (#4165)

OPENAI_BASE_URL was written to .env AND config.yaml, creating a dual-source
confusion. Users (especially Docker) would see the URL in .env and assume
that's where all config lives, then wonder why LLM_MODEL in .env didn't work.

Changes:
- Remove all 27 save_env_value("OPENAI_BASE_URL", ...) calls across main.py,
  setup.py, and tools_config.py
- Remove OPENAI_BASE_URL env var reading from runtime_provider.py, cli.py,
  models.py, and gateway/run.py
- Remove LLM_MODEL/HERMES_MODEL env var reading from gateway/run.py and
  auxiliary_client.py — config.yaml model.default is authoritative
- Vision base URL now saved to config.yaml auxiliary.vision.base_url
  (both setup wizard and tools_config paths)
- Tests updated to set config values instead of env vars

Convention enforced: .env is for SECRETS only (API keys). All other
configuration (model names, base URLs, provider selection) lives
exclusively in config.yaml.

tests/test_cli_provider_resolution.py

@@ -467,6 +467,6 @@ def test_model_flow_custom_saves_verified_v1_base_url(monkeypatch, capsys):
     output = capsys.readouterr().out
 
     assert "Saving the working base URL instead" in output
-    assert saved_env["OPENAI_BASE_URL"] == "http://localhost:8000/v1"
-    assert saved_env["OPENAI_API_KEY"] == "local-key"
+    # OPENAI_BASE_URL is no longer saved to .env — config.yaml is authoritative
+    assert "OPENAI_BASE_URL" not in saved_env
     assert saved_env["MODEL"] == "llm"