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>

website/docs/user-guide/messaging/index.md

@@ -1,7 +1,7 @@
 ---
 sidebar_position: 1
 title: "Messaging Gateway"
-description: "Chat with Hermes from Telegram, Discord, Slack, WhatsApp, Signal, SMS, Email, Home Assistant, Mattermost, Matrix, DingTalk, or your browser — architecture and setup overview"
+description: "Chat with Hermes from Telegram, Discord, Slack, WhatsApp, Signal, SMS, Email, Home Assistant, Mattermost, Matrix, DingTalk, or any OpenAI-compatible frontend via the API server — architecture and setup overview"
 ---
 
 # Messaging Gateway
@@ -27,6 +27,7 @@ flowchart TB
             mm[Mattermost]
             mx[Matrix]
             dt[DingTalk]
+            api["API Server<br/>(OpenAI-compatible)"]
         end
 
         store["Session store<br/>per chat"]
@@ -45,6 +46,7 @@ flowchart TB
     mm --> store
     mx --> store
     dt --> store
+    api --> store
     store --> agent
     cron --> store
 ```
@@ -306,6 +308,7 @@ Each platform has its own toolset:
 | Mattermost | `hermes-mattermost` | Full tools including terminal |
 | Matrix | `hermes-matrix` | Full tools including terminal |
 | DingTalk | `hermes-dingtalk` | Full tools including terminal |
+| API Server | `hermes` (default) | Full tools including terminal |
 
 ## Next Steps
 
@@ -320,3 +323,4 @@ Each platform has its own toolset:
 - [Mattermost Setup](mattermost.md)
 - [Matrix Setup](matrix.md)
 - [DingTalk Setup](dingtalk.md)
+- [Open WebUI + API Server](open-webui.md)