P3 stretch goal for Milestone 40 (v0.21.0). Not required for the milestone; promote only if capacity allows.

Deliver a scheduled daily inbox briefing (overnight/morning) instead of only on-demand. The content already exists — pre_scan_inbox (#1265) produces the per-category card — what's missing is scheduling and push delivery.

Cross-link (avoid duplication): scheduling/delivery depends on the autonomy engine (#555); the briefing content/format overlaps #1525 (tasks in the morning brief). Scope here is the email briefing payload + its scheduled trigger, reusing pre_scan_inbox.

Gap reference: capability gap G5 in the email use-case doc.

Acceptance criteria

• A scheduled trigger produces the pre_scan_inbox briefing without a user prompt.
• Briefing is delivered/pushed (surface TBD with the autonomy engine), not just returned to a live caller.
• Schedule is configurable and off by default.

Test acceptance criteria

• Unit test: the scheduled job invokes pre_scan_inbox and produces the email_pre_scan envelope.
• Unit test: disabled schedule produces no briefing.