gws chat send: add --at flag for scheduled message delivery #199

Feature Request

Add a --at flag to gws chat send to schedule a message for delivery at a future time.

Motivation

Currently gws chat send fires immediately. There's no way to compose a reply and have it sent at a specific time (e.g. next morning). Workarounds require external cron jobs or a separate scheduler, which is friction for a common use case.

Proposed API

# Send at a specific time (RFC3339)
gws chat send --space <space-id> --text "Hello" --at "2026-06-24T09:00:00+03:00"

# Human-friendly relative form (nice to have)
gws chat send --space <space-id> --text "Hello" --at "tomorrow 9am"

Behavior

• If --at is omitted, behavior is unchanged (send immediately).
• If --at is in the past, return an error.
• The CLI should store the scheduled message locally (or via a lightweight daemon/cron entry) and execute the send at the specified time.
• Output on scheduling: confirmation with the resolved send time.

Notes

• Google Chat API has no native message scheduling endpoint, so this would be client-side scheduling (local cron or background process).
• A simple approach: write the pending send to a state file and register a launchd/cron entry for the target time, then clean up after delivery.