feat(09-03): rewrite executeNotify with route_to_user branch and fallback semantics
- Extract dispatchToGlobalChannel helper (backward-compat path unchanged)
- Add dispatchUserRoute: field lookup, resolver dispatch, email→user_id
resolution, mute check, personal channel lookup, send with fallback
- Add fallbackToGlobal: annotates output with user_route_fallback reason
- Mute path (enabled=false) returns success:true/notified:false, no fallback
- Default channel-type order when omitted: ntfy then teams (ROUTE-06)
- Export _INTERNALS test seam following link-discovery.ts precedent
- All five fallback reasons: no_channel, send_failed, user_not_found,
no_field_value, resolver_unknown (ROUTE-03..06)
lorentz
86acc06b16