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.

cli.py

@@ -1124,9 +1124,9 @@ class HermesCLI:
         self.acp_args: list[str] = []
         self.base_url = (
             base_url
-            or os.getenv("OPENAI_BASE_URL")
-            or os.getenv("OPENROUTER_BASE_URL", CLI_CONFIG["model"]["base_url"])
-        )
+            or CLI_CONFIG["model"].get("base_url", "")
+            or os.getenv("OPENROUTER_BASE_URL", "")
+        ) or None
         # Match key to resolved base_url: OpenRouter URL → prefer OPENROUTER_API_KEY,
         # custom endpoint → prefer OPENAI_API_KEY (issue #560).
         # Note: _ensure_runtime_credentials() re-resolves this before first use.
@@ -3239,7 +3239,7 @@ class HermesCLI:
                         print(f"      {mid}{current_marker}")
                 elif p["id"] == "custom":
                     from hermes_cli.models import _get_custom_base_url
-                    custom_url = _get_custom_base_url() or os.getenv("OPENAI_BASE_URL", "")
+                    custom_url = _get_custom_base_url()
                     if custom_url:
                         print(f"      endpoint: {custom_url}")
                     if is_active: