feat: add Codex fast mode toggle (/fast command)

Add /fast slash command to toggle OpenAI Codex service_tier between
normal and priority ('fast') inference. Only exposed for models
registered in _FAST_MODE_BACKEND_CONFIG (currently gpt-5.4).

- Registry-based backend config for extensibility
- Dynamic command visibility (hidden from help/autocomplete for
  non-supported models) via command_filter on SlashCommandCompleter
- service_tier flows through request_overrides from route resolution
- Omit max_output_tokens for Codex backend (rejects it)
- Persists to config.yaml under agent.service_tier

Salvage cleanup: removed simple_term_menu/input() menu (banned),
bare /fast now shows status like /reasoning. Removed redundant
override resolution in _build_api_kwargs — single source of truth
via request_overrides from route.

Co-authored-by: Hermes Agent <hermes@nousresearch.com>

run_agent.py

@@ -500,6 +500,8 @@ class AIAgent:
         status_callback: callable = None,
         max_tokens: int = None,
         reasoning_config: Dict[str, Any] = None,
+        service_tier: str = None,
+        request_overrides: Dict[str, Any] = None,
         prefill_messages: List[Dict[str, Any]] = None,
         platform: str = None,
         user_id: str = None,
@@ -662,6 +664,8 @@ class AIAgent:
         # Model response configuration
         self.max_tokens = max_tokens  # None = use model default
         self.reasoning_config = reasoning_config  # None = use default (medium for OpenRouter)
+        self.service_tier = service_tier
+        self.request_overrides = dict(request_overrides or {})
         self.prefill_messages = prefill_messages or []  # Prefilled conversation turns
         
         # Anthropic prompt caching: auto-enabled for Claude models via OpenRouter.
@@ -3343,7 +3347,7 @@ class AIAgent:
         allowed_keys = {
             "model", "instructions", "input", "tools", "store",
             "reasoning", "include", "max_output_tokens", "temperature",
-            "tool_choice", "parallel_tool_calls", "prompt_cache_key",
+            "tool_choice", "parallel_tool_calls", "prompt_cache_key", "service_tier",
         }
         normalized: Dict[str, Any] = {
             "model": model,
@@ -3361,6 +3365,9 @@ class AIAgent:
         include = api_kwargs.get("include")
         if isinstance(include, list):
             normalized["include"] = include
+        service_tier = api_kwargs.get("service_tier")
+        if isinstance(service_tier, str) and service_tier.strip():
+            normalized["service_tier"] = service_tier.strip()
 
         # Pass through max_output_tokens and temperature
         max_output_tokens = api_kwargs.get("max_output_tokens")
@@ -5464,6 +5471,10 @@ class AIAgent:
                 "models.github.ai" in self.base_url.lower()
                 or "api.githubcopilot.com" in self.base_url.lower()
             )
+            is_codex_backend = (
+                self.provider == "openai-codex"
+                or "chatgpt.com/backend-api/codex" in self.base_url.lower()
+            )
 
             # Resolve reasoning effort: config > default (medium)
             reasoning_effort = "medium"
@@ -5501,7 +5512,10 @@ class AIAgent:
             elif not is_github_responses:
                 kwargs["include"] = []
 
-            if self.max_tokens is not None:
+            if self.request_overrides:
+                kwargs.update(self.request_overrides)
+
+            if self.max_tokens is not None and not is_codex_backend:
                 kwargs["max_output_tokens"] = self.max_tokens
 
             return kwargs