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

gateway/run.py

@@ -1544,7 +1544,7 @@ class GatewayRunner:
         config_path = _hermes_home / 'config.yaml'
 
         # Resolve current model and provider from config
-        current = os.getenv("HERMES_MODEL") or os.getenv("LLM_MODEL") or "anthropic/claude-opus-4.6"
+        current = os.getenv("HERMES_MODEL") or "anthropic/claude-opus-4.6"
         current_provider = "openrouter"
         try:
             if config_path.exists():
@@ -1999,7 +1999,7 @@ class GatewayRunner:
                 return
 
             # Read model from config (same as _run_agent)
-            model = os.getenv("HERMES_MODEL") or os.getenv("LLM_MODEL") or "anthropic/claude-opus-4.6"
+            model = os.getenv("HERMES_MODEL") or "anthropic/claude-opus-4.6"
             try:
                 import yaml as _y
                 _cfg_path = _hermes_home / "config.yaml"
@@ -3093,7 +3093,7 @@ class GatewayRunner:
             except Exception:
                 pass
 
-            model = os.getenv("HERMES_MODEL") or os.getenv("LLM_MODEL") or "anthropic/claude-opus-4.6"
+            model = os.getenv("HERMES_MODEL") or "anthropic/claude-opus-4.6"
 
             try:
                 import yaml as _y