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

hermes_cli/dump.py

@@ -119,6 +119,7 @@ def _configured_platforms() -> list[str]:
         "dingtalk": "DINGTALK_CLIENT_ID",
         "feishu": "FEISHU_APP_ID",
         "wecom": "WECOM_BOT_ID",
+        "weixin": "WEIXIN_ACCOUNT_ID",
     }
     return [name for name, env in checks.items() if os.getenv(env)]