feat(gateway): add DingTalk platform adapter (#1685)

Add DingTalk as a messaging platform using the dingtalk-stream SDK
for real-time message reception via Stream Mode (no webhook needed).
Replies are sent via session webhook using markdown format.

Features:
- Stream Mode connection (long-lived WebSocket, no public URL needed)
- Text and rich text message support
- DM and group chat support
- Message deduplication with 5-minute window
- Auto-reconnection with exponential backoff
- Session webhook caching for reply routing

Configuration:
  export DINGTALK_CLIENT_ID=your-app-key
  export DINGTALK_CLIENT_SECRET=your-app-secret

  # or in config.yaml:
  platforms:
    dingtalk:
      enabled: true
      extra:
        client_id: your-app-key
        client_secret: your-app-secret

Files:
- gateway/platforms/dingtalk.py (340 lines) — adapter implementation
- gateway/config.py — add DINGTALK to Platform enum
- gateway/run.py — add DingTalk to _create_adapter
- hermes_cli/config.py — add env vars to _EXTRA_ENV_KEYS
- hermes_cli/tools_config.py — add dingtalk to PLATFORMS
- tests/gateway/test_dingtalk.py — 21 tests

hermes_cli/tools_config.py

@@ -110,6 +110,7 @@ PLATFORMS = {
     "whatsapp": {"label": "📱 WhatsApp",   "default_toolset": "hermes-whatsapp"},
     "signal":   {"label": "📡 Signal",     "default_toolset": "hermes-signal"},
     "email":    {"label": "📧 Email",      "default_toolset": "hermes-email"},
+    "dingtalk": {"label": "💬 DingTalk",   "default_toolset": "hermes-dingtalk"},
 }