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.)

gateway/session.py

@@ -193,6 +193,7 @@ _PII_SAFE_PLATFORMS = frozenset({
     Platform.WHATSAPP,
     Platform.SIGNAL,
     Platform.TELEGRAM,
+    Platform.BLUEBUBBLES,
 })
 """Platforms where user IDs can be safely redacted (no in-message mention system
 that requires raw IDs).  Discord is excluded because mentions use ``<@user_id>``