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/architecture.md

@@ -118,7 +118,7 @@ hermes-agent/
 │   ├── builtin_hooks/        # Always-registered hooks
 │   └── platforms/            # 15 adapters: telegram, discord, slack, whatsapp,
 │                             #   signal, matrix, mattermost, email, sms,
-│                             #   dingtalk, feishu, wecom, bluebubbles, homeassistant, webhook
+│                             #   dingtalk, feishu, wecom, weixin, bluebubbles, homeassistant, webhook
 │
 ├── acp_adapter/              # ACP server (VS Code / Zed / JetBrains)
 ├── cron/                     # Scheduler (jobs.py, scheduler.py)