fix: complete Weixin platform parity audit — 16 missing integration points

Systematic audit found Weixin missing from:

Code:
- gateway/run.py: early WEIXIN_ALLOW_ALL_USERS env check
- gateway/platforms/webhook.py: cross-platform delivery routing
- hermes_cli/dump.py: platform detection for config export
- hermes_cli/setup.py: hermes setup wizard platform list + _setup_weixin
- hermes_cli/skills_config.py: platform labels for skills config UI

Docs (11 pages):
- developer-guide/architecture.md: platform adapter listing
- developer-guide/cron-internals.md: delivery target table
- developer-guide/gateway-internals.md: file tree
- guides/cron-troubleshooting.md: supported platforms list
- integrations/index.md: platform links
- reference/toolsets-reference.md: toolset table
- user-guide/configuration.md: platform keys for tool_progress
- user-guide/features/cron.md: delivery target table
- user-guide/messaging/index.md: intro text, feature table,
  mermaid diagram, toolset table, setup links
- user-guide/messaging/webhooks.md: deliver field + routing table
- user-guide/sessions.md: platform identifiers table

website/docs/developer-guide/cron-internals.md

@@ -169,6 +169,7 @@ Cron job results can be delivered to any supported platform:
 | DingTalk | `dingtalk` | Deliver to DingTalk |
 | Feishu | `feishu` | Deliver to Feishu |
 | WeCom | `wecom` | Deliver to WeCom |
+| Weixin | `weixin` | Deliver to Weixin (WeChat) |
 | BlueBubbles | `bluebubbles` | Deliver to iMessage via BlueBubbles |
 
 For Telegram topics, use the format `telegram:<chat_id>:<thread_id>` (e.g., `telegram:-1001234567890:17585`).