feat(providers): add Volcengine and BytePlus support

Based on PR #8952 by @Maaannnn.

Adds Volcengine and BytePlus as first-class providers, each with standard
and Coding Plan model catalogs. The model prefix (volcengine/ vs
volcengine-coding-plan/) determines the runtime base URL automatically.

- New hermes_cli/provider_contracts.py centralises all constants
- ProviderConfig entries in auth.py with api_key auth
- Model catalogs, aliases, and provider ordering in models.py/providers.py
- Auxiliary client entries and context window resolution
- gateway /provider command detects known Volcengine/BytePlus endpoints
- Comprehensive tests and docs update

hermes_cli/main.py

@@ -1570,6 +1570,8 @@ def select_provider_and_model(args=None):
         _model_flow_stepfun(config, current_model)
     elif selected_provider == "bedrock":
         _model_flow_bedrock(config, current_model)
+    elif selected_provider in ("volcengine", "byteplus"):
+        _model_flow_contract_provider(config, selected_provider, current_model)
     elif selected_provider in (
         "gemini",
         "deepseek",
@@ -1954,7 +1956,7 @@ def _aux_flow_custom_endpoint(task: str, task_cfg: dict) -> None:
     print(f"{display_name}: custom ({short_url})" + (f" · {model}" if model else ""))
 
 
-def _prompt_provider_choice(choices, *, default=0):
+def _prompt_provider_choice(choices, *, default=0, title="Select provider:"):
     """Show provider selection menu with curses arrow-key navigation.
 
     Falls back to a numbered list when curses is unavailable (e.g. piped
@@ -1963,8 +1965,7 @@ def _prompt_provider_choice(choices, *, default=0):
     """
     try:
         from hermes_cli.setup import _curses_prompt_choice
-
-        idx = _curses_prompt_choice("Select provider:", choices, default)
+        idx = _curses_prompt_choice(title, choices, default)
         if idx >= 0:
             print()
             return idx
@@ -1972,7 +1973,7 @@ def _prompt_provider_choice(choices, *, default=0):
         pass
 
     # Fallback: numbered list
-    print("Select provider:")
+    print(title)
     for i, c in enumerate(choices, 1):
         marker = "→" if i - 1 == default else " "
         print(f"  {marker} {i}. {c}")
@@ -2944,6 +2945,10 @@ def _model_flow_named_custom(config, provider_info):
 
 # Curated model lists for direct API-key providers — single source in models.py
 from hermes_cli.models import _PROVIDER_MODELS
+from hermes_cli.provider_contracts import (
+    base_url_for_provider_model,
+    provider_models,
+)
 
 
 def _current_reasoning_effort(config) -> str:
@@ -4033,6 +4038,70 @@ def _model_flow_api_key_provider(config, provider_id, current_model=""):
         print("No change.")
 
 
+def _model_flow_contract_provider(config, provider_id, current_model=""):
+    """Provider flow for Volcengine / BytePlus contract-backed catalogs."""
+    from hermes_cli.auth import (
+        PROVIDER_REGISTRY,
+        _prompt_model_selection,
+        _save_model_choice,
+        deactivate_provider,
+    )
+    from hermes_cli.config import get_env_value, load_config, save_config, save_env_value
+
+    pconfig = PROVIDER_REGISTRY[provider_id]
+    key_env = pconfig.api_key_env_vars[0] if pconfig.api_key_env_vars else ""
+    existing_key = ""
+    for env_var in pconfig.api_key_env_vars:
+        existing_key = get_env_value(env_var) or os.getenv(env_var, "")
+        if existing_key:
+            break
+
+    if not existing_key:
+        print(f"No {pconfig.name} API key configured.")
+        if key_env:
+            try:
+                import getpass
+
+                new_key = getpass.getpass(f"{key_env} (or Enter to cancel): ").strip()
+            except (KeyboardInterrupt, EOFError):
+                print()
+                return
+            if not new_key:
+                print("Cancelled.")
+                return
+            save_env_value(key_env, new_key)
+            print("API key saved.")
+            print()
+    else:
+        print(f"  {pconfig.name} API key: {existing_key[:8]}... ✓")
+        print()
+
+    model_list = provider_models(provider_id)
+    if not model_list:
+        print(f"No curated model catalog found for {pconfig.name}.")
+        return
+
+    selected = _prompt_model_selection(model_list, current_model=current_model)
+    if not selected:
+        print("No change.")
+        return
+
+    _save_model_choice(selected)
+
+    cfg = load_config()
+    model = cfg.get("model")
+    if not isinstance(model, dict):
+        model = {"default": model} if model else {}
+        cfg["model"] = model
+    model["provider"] = provider_id
+    model["base_url"] = base_url_for_provider_model(provider_id, selected)
+    model.pop("api_mode", None)
+    save_config(cfg)
+    deactivate_provider()
+
+    print(f"Default model set to: {selected} (via {pconfig.name})")
+
+
 def _run_anthropic_oauth_flow(save_env_value):
     """Run the Claude OAuth setup-token flow. Returns True if credentials were saved."""
     from agent.anthropic_adapter import (