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>

hermes_cli/config.py

@@ -332,6 +332,14 @@ DEFAULT_CONFIG = {
         "auto_thread": True,           # Auto-create threads on @mention in channels (like Slack)
     },
 
+    # WhatsApp platform settings (gateway mode)
+    "whatsapp": {
+        # Reply prefix prepended to every outgoing WhatsApp message.
+        # Default (None) uses the built-in "⚕ *Hermes Agent*" header.
+        # Set to "" (empty string) to disable the header entirely.
+        # Supports \n for newlines, e.g. "🤖 *My Bot*\n──────\n"
+    },
+
     # Approval mode for dangerous commands:
     #   manual — always prompt the user (default)
     #   smart  — use auxiliary LLM to auto-approve low-risk commands, prompt for high-risk
@@ -364,7 +372,7 @@ DEFAULT_CONFIG = {
     },
 
     # Config schema version - bump this when adding new required fields
-    "_config_version": 9,
+    "_config_version": 10,
 }
 
 # =============================================================================
@@ -767,6 +775,38 @@ OPTIONAL_ENV_VARS = {
         "category": "messaging",
         "advanced": True,
     },
+    "API_SERVER_ENABLED": {
+        "description": "Enable the OpenAI-compatible API server (true/false). Allows frontends like Open WebUI, LobeChat, etc. to connect.",
+        "prompt": "Enable API server (true/false)",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+        "advanced": True,
+    },
+    "API_SERVER_KEY": {
+        "description": "Bearer token for API server authentication. If empty, all requests are allowed (local use only).",
+        "prompt": "API server auth key (optional)",
+        "url": None,
+        "password": True,
+        "category": "messaging",
+        "advanced": True,
+    },
+    "API_SERVER_PORT": {
+        "description": "Port for the API server (default: 8642).",
+        "prompt": "API server port",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+        "advanced": True,
+    },
+    "API_SERVER_HOST": {
+        "description": "Host/bind address for the API server (default: 127.0.0.1). Use 0.0.0.0 for network access — requires API_SERVER_KEY for security.",
+        "prompt": "API server host",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+        "advanced": True,
+    },
 
     # ── Agent settings ──
     "MESSAGING_CWD": {