refactor: remove LLM_MODEL env var dependency — config.yaml is sole source of truth

Model selection now comes exclusively from config.yaml (set via
'hermes model' or 'hermes setup'). The LLM_MODEL env var is no longer
read or written anywhere in production code.

Why: env vars are per-process/per-user and would conflict in
multi-agent or multi-tenant setups. Config.yaml is file-based and
can be scoped per-user or eventually per-session.

Changes:
- cli.py: Read model from CLI_CONFIG only, not LLM_MODEL/OPENAI_MODEL
- hermes_cli/auth.py: _save_model_choice() no longer writes LLM_MODEL
  to .env
- hermes_cli/setup.py: Remove 12 save_env_value('LLM_MODEL', ...)
  calls from all provider setup flows
- gateway/run.py: Remove LLM_MODEL fallback (HERMES_MODEL still works
  for gateway process runtime)
- cron/scheduler.py: Same
- agent/auxiliary_client.py: Remove LLM_MODEL from custom endpoint
  model detection

tests/test_cli_provider_resolution.py

@@ -197,21 +197,28 @@ def test_codex_provider_replaces_incompatible_default_model(monkeypatch):
     assert shell.model == "gpt-5.2-codex"
 
 
-def test_codex_provider_trusts_explicit_envvar_model(monkeypatch):
-    """When the user explicitly sets LLM_MODEL, we trust their choice and
-    let the API be the judge — even if it's a non-OpenAI model.  Only
-    provider prefixes are stripped; the bare model passes through."""
+def test_codex_provider_uses_config_model(monkeypatch):
+    """Model comes from config.yaml, not LLM_MODEL env var.
+    Config.yaml is the single source of truth to avoid multi-agent conflicts."""
     cli = _import_cli()
 
-    monkeypatch.setenv("LLM_MODEL", "claude-opus-4-6")
+    # LLM_MODEL env var should be IGNORED (even if set)
+    monkeypatch.setenv("LLM_MODEL", "should-be-ignored")
     monkeypatch.delenv("OPENAI_MODEL", raising=False)
 
+    # Set model via config
+    monkeypatch.setitem(cli.CLI_CONFIG, "model", {
+        "default": "gpt-5.2-codex",
+        "provider": "openai-codex",
+        "base_url": "https://chatgpt.com/backend-api/codex",
+    })
+
     def _runtime_resolve(**kwargs):
         return {
             "provider": "openai-codex",
             "api_mode": "codex_responses",
             "base_url": "https://chatgpt.com/backend-api/codex",
-            "api_key": "test-key",
+            "api_key": "fake-codex-token",
             "source": "env/config",
         }
 
@@ -220,11 +227,12 @@ def test_codex_provider_trusts_explicit_envvar_model(monkeypatch):
 
     shell = cli.HermesCLI(compact=True, max_turns=1)
 
-    assert shell._model_is_default is False
     assert shell._ensure_runtime_credentials() is True
     assert shell.provider == "openai-codex"
-    # User explicitly chose this model — it passes through untouched
-    assert shell.model == "claude-opus-4-6"
+    # Model from config (may be normalized by codex provider logic)
+    assert "codex" in shell.model.lower()
+    # LLM_MODEL env var is NOT used
+    assert shell.model != "should-be-ignored"
 
 
 def test_codex_provider_preserves_explicit_codex_model(monkeypatch):