docs: add platform adapter developer guide + WeCom Callback docs (#7969)

Add the missing 'Adding a Platform Adapter' developer guide — a
comprehensive step-by-step checklist covering all 20+ integration
points (enum, adapter, config, runner, CLI, tools, toolsets, cron,
webhooks, tests, and docs). Includes common patterns for long-poll,
callback/webhook, and token-lock adapters with reference implementations.

Also adds full docs coverage for the WeCom Callback platform:
- New docs page: user-guide/messaging/wecom-callback.md
- Environment variables reference (9 WECOM_CALLBACK_* vars)
- Toolsets reference (hermes-wecom-callback)
- Messaging index (comparison table, architecture diagram, toolsets,
  security, next-steps links)
- Integrations index listing
- Sidebar entries for both new pages

hermes_cli/status.py

@@ -302,6 +302,7 @@ def show_status(args):
         "DingTalk": ("DINGTALK_CLIENT_ID", None),
         "Feishu": ("FEISHU_APP_ID", "FEISHU_HOME_CHANNEL"),
         "WeCom": ("WECOM_BOT_ID", "WECOM_HOME_CHANNEL"),
+        "WeCom Callback": ("WECOM_CALLBACK_CORP_ID", None),
         "Weixin": ("WEIXIN_ACCOUNT_ID", "WEIXIN_HOME_CHANNEL"),
         "BlueBubbles": ("BLUEBUBBLES_SERVER_URL", "BLUEBUBBLES_HOME_CHANNEL"),
     }