fix(bluebubbles): add missing integration points and documentation (#6460)

- hermes_cli/skills_config.py: add platform label for per-platform skill config
- gateway/session.py: add to PII-safe platforms (no mention system)
- website/docs/user-guide/messaging/bluebubbles.md: full setup guide
- website/sidebars.ts: sidebar navigation entry
- 10 docs pages: add BlueBubbles to all platform enumerations
  (env vars, toolsets, cron delivery, gateway internals, etc.)

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

@@ -160,6 +160,7 @@ gateway/platforms/
 ├── dingtalk.py          # DingTalk WebSocket
 ├── feishu.py            # Feishu/Lark WebSocket or webhook
 ├── wecom.py             # WeCom (WeChat Work) callback
+├── bluebubbles.py       # Apple iMessage via BlueBubbles macOS server
 ├── webhook.py           # Inbound/outbound webhook adapter
 ├── api_server.py        # REST API server adapter
 └── homeassistant.py     # Home Assistant conversation integration