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/platforms.py

@@ -33,6 +33,7 @@ PLATFORMS: OrderedDict[str, PlatformInfo] = OrderedDict([
     ("dingtalk",       PlatformInfo(label="💬 DingTalk",        default_toolset="hermes-dingtalk")),
     ("feishu",         PlatformInfo(label="🪽 Feishu",          default_toolset="hermes-feishu")),
     ("wecom",          PlatformInfo(label="💬 WeCom",           default_toolset="hermes-wecom")),
+    ("wecom_callback", PlatformInfo(label="💬 WeCom Callback",  default_toolset="hermes-wecom-callback")),
     ("weixin",         PlatformInfo(label="💬 Weixin",          default_toolset="hermes-weixin")),
     ("webhook",        PlatformInfo(label="🔗 Webhook",         default_toolset="hermes-webhook")),
     ("api_server",     PlatformInfo(label="🌐 API Server",      default_toolset="hermes-api-server")),