feat: OpenAI-compatible API server + WhatsApp configurable reply prefix (#1756)

* feat: OpenAI-compatible API server platform adapter

Salvaged from PR #956, updated for current main.

Adds an HTTP API server as a gateway platform adapter that exposes
hermes-agent via the OpenAI Chat Completions and Responses APIs.
Any OpenAI-compatible frontend (Open WebUI, LobeChat, LibreChat,
AnythingLLM, NextChat, ChatBox, etc.) can connect by pointing at
http://localhost:8642/v1.

Endpoints:
- POST /v1/chat/completions  — stateless Chat Completions API
- POST /v1/responses         — stateful Responses API with chaining
- GET  /v1/responses/{id}    — retrieve stored response
- DELETE /v1/responses/{id}  — delete stored response
- GET  /v1/models            — list hermes-agent as available model
- GET  /health               — health check

Features:
- Real SSE streaming via stream_delta_callback (uses main's streaming)
- In-memory LRU response store for Responses API conversation chaining
- Named conversations via 'conversation' parameter
- Bearer token auth (optional, via API_SERVER_KEY)
- CORS support for browser-based frontends
- System prompt layering (frontend system messages on top of core)
- Real token usage tracking in responses

Integration points:
- Platform.API_SERVER in gateway/config.py
- _create_adapter() branch in gateway/run.py
- API_SERVER_* env vars in hermes_cli/config.py
- Env var overrides in gateway/config.py _apply_env_overrides()

Changes vs original PR #956:
- Removed streaming infrastructure (already on main via stream_consumer.py)
- Removed Telegram reply_to_mode (separate feature, not included)
- Updated _resolve_model() -> _resolve_gateway_model()
- Updated stream_callback -> stream_delta_callback
- Updated connect()/disconnect() to use _mark_connected()/_mark_disconnected()
- Adapted to current Platform enum (includes MATTERMOST, MATRIX, DINGTALK)

Tests: 72 new tests, all passing
Docs: API server guide, Open WebUI integration guide, env var reference

* feat(whatsapp): make reply prefix configurable via config.yaml

Reworked from PR #1764 (ifrederico) to use config.yaml instead of .env.

The WhatsApp bridge prepends a header to every outgoing message.
This was hardcoded to '⚕ *Hermes Agent*'. Users can now customize
or disable it via config.yaml:

  whatsapp:
    reply_prefix: ''                     # disable header
    reply_prefix: '🤖 *My Bot*\n───\n'  # custom prefix

How it works:
- load_gateway_config() reads whatsapp.reply_prefix from config.yaml
  and stores it in PlatformConfig.extra['reply_prefix']
- WhatsAppAdapter reads it from config.extra at init
- When spawning bridge.js, the adapter passes it as
  WHATSAPP_REPLY_PREFIX in the subprocess environment
- bridge.js handles undefined (default), empty (no header),
  or custom values with \\n escape support
- Self-chat echo suppression uses the configured prefix

Also fixes _config_version: was 9 but ENV_VARS_BY_VERSION had a
key 10 (TAVILY_API_KEY), so existing users at v9 would never be
prompted for Tavily. Bumped to 10 to close the gap. Added a
regression test to prevent this from happening again.

Credit: ifrederico (PR #1764) for the bridge.js implementation
and the config version gap discovery.

---------

Co-authored-by: Test <test@test.com>

gateway/config.py

@@ -46,6 +46,7 @@ class Platform(Enum):
     EMAIL = "email"
     SMS = "sms"
     DINGTALK = "dingtalk"
+    API_SERVER = "api_server"
 
 
 @dataclass
@@ -238,6 +239,9 @@ class GatewayConfig:
             # SMS uses api_key (Twilio auth token) — SID checked via env
             elif platform == Platform.SMS and os.getenv("TWILIO_ACCOUNT_SID"):
                 connected.append(platform)
+            # API Server uses enabled flag only (no token needed)
+            elif platform == Platform.API_SERVER:
+                connected.append(platform)
         return connected
     
     def get_home_channel(self, platform: Platform) -> Optional[HomeChannel]:
@@ -416,6 +420,13 @@ def load_gateway_config() -> GatewayConfig:
                     os.environ["DISCORD_FREE_RESPONSE_CHANNELS"] = str(frc)
                 if "auto_thread" in discord_cfg and not os.getenv("DISCORD_AUTO_THREAD"):
                     os.environ["DISCORD_AUTO_THREAD"] = str(discord_cfg["auto_thread"]).lower()
+
+            # Bridge whatsapp settings from config.yaml into platform config
+            whatsapp_cfg = yaml_cfg.get("whatsapp", {})
+            if isinstance(whatsapp_cfg, dict) and "reply_prefix" in whatsapp_cfg:
+                if Platform.WHATSAPP not in config.platforms:
+                    config.platforms[Platform.WHATSAPP] = PlatformConfig()
+                config.platforms[Platform.WHATSAPP].extra["reply_prefix"] = whatsapp_cfg["reply_prefix"]
     except Exception:
         pass
 
@@ -634,6 +645,25 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
                 name=os.getenv("SMS_HOME_CHANNEL_NAME", "Home"),
             )
 
+    # API Server
+    api_server_enabled = os.getenv("API_SERVER_ENABLED", "").lower() in ("true", "1", "yes")
+    api_server_key = os.getenv("API_SERVER_KEY", "")
+    api_server_port = os.getenv("API_SERVER_PORT")
+    api_server_host = os.getenv("API_SERVER_HOST")
+    if api_server_enabled or api_server_key:
+        if Platform.API_SERVER not in config.platforms:
+            config.platforms[Platform.API_SERVER] = PlatformConfig()
+        config.platforms[Platform.API_SERVER].enabled = True
+        if api_server_key:
+            config.platforms[Platform.API_SERVER].extra["key"] = api_server_key
+        if api_server_port:
+            try:
+                config.platforms[Platform.API_SERVER].extra["port"] = int(api_server_port)
+            except ValueError:
+                pass
+        if api_server_host:
+            config.platforms[Platform.API_SERVER].extra["host"] = api_server_host
+
     # Session settings
     idle_minutes = os.getenv("SESSION_IDLE_MINUTES")
     if idle_minutes: