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>

tests/run_agent/test_run_agent_codex_responses.py

@@ -648,6 +648,15 @@ def test_preflight_codex_api_kwargs_allows_reasoning_and_temperature(monkeypatch
     assert result["max_output_tokens"] == 4096
 
 
+def test_preflight_codex_api_kwargs_allows_service_tier(monkeypatch):
+    agent = _build_agent(monkeypatch)
+    kwargs = _codex_request_kwargs()
+    kwargs["service_tier"] = "priority"
+
+    result = agent._preflight_codex_api_kwargs(kwargs)
+    assert result["service_tier"] == "priority"
+
+
 def test_run_conversation_codex_replay_payload_keeps_call_id(monkeypatch):
     agent = _build_agent(monkeypatch)
     responses = [_codex_tool_call_response(), _codex_message_response("done")]