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/guides/cron-troubleshooting.md

@@ -70,7 +70,7 @@ Delivery targets are case-sensitive and require the correct platform to be confi
 | `local` | Write access to `~/.hermes/cron/output/` |
 | `origin` | Delivers to the chat where the job was created |
 
-Other supported platforms include `mattermost`, `homeassistant`, `dingtalk`, `feishu`, `wecom`, `bluebubbles`, and `webhook`. You can also target a specific chat with `platform:chat_id` syntax (e.g., `telegram:-1001234567890`).
+Other supported platforms include `mattermost`, `homeassistant`, `dingtalk`, `feishu`, `wecom`, `weixin`, `bluebubbles`, and `webhook`. You can also target a specific chat with `platform:chat_id` syntax (e.g., `telegram:-1001234567890`).
 
 If delivery fails, the job still runs — it just won't send anywhere. Check `hermes cron list` for updated `last_error` field (if available).