feat(notifications): daily digest mode instead of per-run

[bjridicodes]

Batch notifications into one daily summary instead of alerting after every run. Important once cron runs every 6h — prevents Telegram spam.

[bjridicodes]

Relationships

Depends on:

• feat(tracking): application status states and CLI update tool #34 — digest summary includes status context (applied, interviewing)

Blocks:

• feat(scheduling): cron job setup + smart triggers (Phase 7 foundation) #36 — cron scheduling needs digest mode to avoid notification spam on frequent runs