Await the session pool in notify and three neighbours ...

PgSessionStore._get_pool() is async, and four call sites passed its coroutine
straight into a store constructor, so the first query inside died with
"'coroutine' object has no attribute 'fetchrow'": notify always failed, the
reaction runner never got past reading its settings, synapse_instructions was
unusable, and the BYOK resolver caught the AttributeError and silently fell
back to the default credential.

The tests missed it because each one mocked the store or the pool provider
away; the new ones run on a fake asyncpg pool with nothing faked below the tool.

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)