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/user-guide/features/cron.md

@@ -202,6 +202,7 @@ When scheduling jobs, you specify where the output goes:
 | `"dingtalk"` | DingTalk | |
 | `"feishu"` | Feishu/Lark | |
 | `"wecom"` | WeCom | |
+| `"weixin"` | Weixin (WeChat) | |
 | `"bluebubbles"` | BlueBubbles (iMessage) | |
 
 The agent's final response is automatically delivered. You do not need to call `send_message` in the cron prompt.