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/hermes_cli/test_commands.py

@@ -446,6 +446,13 @@ class TestSubcommands:
         assert "show" in subs
         assert "hide" in subs
 
+    def test_fast_has_subcommands(self):
+        assert "/fast" in SUBCOMMANDS
+        subs = SUBCOMMANDS["/fast"]
+        assert "fast" in subs
+        assert "normal" in subs
+        assert "status" in subs
+
     def test_voice_has_subcommands(self):
         assert "/voice" in SUBCOMMANDS
         assert "on" in SUBCOMMANDS["/voice"]
@@ -474,6 +481,20 @@ class TestSubcommandCompletion:
         assert "high" in texts
         assert "show" in texts
 
+    def test_fast_subcommand_completion_after_space(self):
+        completions = _completions(SlashCommandCompleter(), "/fast ")
+        texts = {c.text for c in completions}
+        assert "fast" in texts
+        assert "normal" in texts
+
+    def test_fast_command_filtered_out_when_unavailable(self):
+        completions = _completions(
+            SlashCommandCompleter(command_filter=lambda cmd: cmd != "/fast"),
+            "/fa",
+        )
+        texts = {c.text for c in completions}
+        assert "fast" not in texts
+
     def test_subcommand_prefix_filters(self):
         """Typing '/reasoning sh' should only show 'show'."""
         completions = _completions(SlashCommandCompleter(), "/reasoning sh")
@@ -527,6 +548,13 @@ class TestGhostText:
         """/reasoning sh → 'ow'"""
         assert _suggestion("/reasoning sh") == "ow"
 
+    def test_fast_subcommand_suggestion(self):
+        assert _suggestion("/fast f") == "ast"
+
+    def test_fast_subcommand_suggestion_hidden_when_filtered(self):
+        completer = SlashCommandCompleter(command_filter=lambda cmd: cmd != "/fast")
+        assert _suggestion("/fa", completer=completer) is None
+
     def test_no_suggestion_for_non_slash(self):
         assert _suggestion("hello") is None