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

website/docs/reference/toolsets-reference.md

@@ -103,6 +103,7 @@ Platform toolsets define the complete tool configuration for a deployment target
 | `hermes-dingtalk` | Same as `hermes-cli`. |
 | `hermes-feishu` | Same as `hermes-cli`. |
 | `hermes-wecom` | Same as `hermes-cli`. |
+| `hermes-wecom-callback` | WeCom callback toolset — enterprise self-built app messaging (full access). |
 | `hermes-weixin` | Same as `hermes-cli`. |
 | `hermes-bluebubbles` | Same as `hermes-cli`. |
 | `hermes-homeassistant` | Same as `hermes-cli`. |