synapse reactions W3: reaction runner, notify tool, gateway hookup ...

- navi/synapse/reactions.py: fire-and-forget reaction run — per-user
  reactions_enabled gate, one-shot dispatcher meta-pass (visible profiles
  only, hidden dispatcher, secretary fallback), special=True session
  headed by the resolved profile, headless agent run via the orchestrator
  run registry, finalise: app push per completion_notify +
  low-level reaction.finished/failed event to Synapse
- navi/synapse/outbound.py: source-side emit (syn_* key from env,
  silent when not configured)
- navi/tools/notify.py: notify tool — message + level (info/warning/
  intervention), both legs per push_target, honest delivered/skipped result
- push/service.py: notify_custom without cooldown
- gateway: verified + non-duplicate delivery schedules the reaction
- profiles: notify added to native tools (7 profiles)