Adds DigestScheduler class that fires digest at configured time (default 07:00)
and routes to rules with trigger_type=schedule and schedule_match=digest.
- DigestScheduler: asyncio task with start/stop lifecycle
- Config: DigestConfig dataclass with schedule and include fields
- Config: schedule_match field on NotificationRuleConfig
- Pipeline: start_pipeline/stop_pipeline async lifecycle functions
- Mesh channels get per-chunk delivery, email/webhook get full text
- 26 new tests covering schedule computation, fire behavior, lifecycle
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
