feat: Add "Schedule for Later" message delivery

[sernst]

Motivation

Asynchronous communication is fundamental to distributed teams and respects work-life balance across time zones. Many teams have members in different regions, and immediate message delivery can interrupt sleep, focus time, or off-hours for collaborators. Slack's "Schedule message" feature has become a standard expectation in team communication platforms, allowing users and agents to compose messages now and deliver them at a time that doesn't interrupt recipients.

Buzz currently lacks this capability, which means:

• Users must manually send messages at specific times or interrupt teammates across time zones
• Agents cannot respect timezone-aware delivery windows for automated messages
• Teams lose the ability to queue communications for working hours without external workarounds

Proposed solution

Add a "Schedule for Later" feature that:

1. Composer integration: Add a "Schedule" button or time-picker in the message composer (alongside "Send now") that lets users specify:

   • Date and time for delivery
   • Optional: recurring schedule or delivery window
   • Display selected delivery time near the send button

2. Agent support: Allow agents to schedule messages via the CLI/API:

   • buzz messages send --channel <uuid> --scheduled-at <ISO8601-timestamp> --content "..."
   • Messages persist in the harness queue across agent restarts

3. Queue and delivery:

   • Store scheduled messages durably (relay-backed or local)
   • Deliver at the specified time automatically
   • Show scheduled messages in a "Scheduled" view before delivery
   • Allow editing/cancellation of pending scheduled messages

4. User experience:

   • Show clear indication in the channel when a message is scheduled vs. sent
   • Display delivery time in local timezone
   • Timezone-aware scheduling for teams across regions (stretch goal)

Alternatives considered

• External scheduling via cron/Lambda: brittle, requires manual setup per team, no UX integration
• Relay-side scheduling microservice: adds operational complexity; feature should live in the client for simplicity
• Draft messages: solves the "compose later" problem but not "deliver later"

Additional context

• Slack's Schedule Message docs
• This mirrors a core feature in Discord, Teams, and other async-communication platforms
• Agents benefit especially: can publish summaries, reminders, and updates outside human working hours
• Duplicate check: None found in open issues