feat(message): schedule Slack messages

[AmethystLiang]

Summary

• Add message send --schedule / --schedule-in backed by Slack chat.scheduleMessage
• Add message scheduled list and message scheduled cancel wrappers
• Document scheduled sends in README, llms.txt, and the bundled agent-slack skill

Review notes

• Verified against current Slack docs for chat.scheduleMessage, chat.scheduledMessages.list, and chat.deleteScheduledMessage
• --attach is rejected for scheduled sends because Slack scheduled messages do not support file uploads
• Natural --schedule-in phrases use the local timezone and absolute schedules require explicit timezone

Verification

• bun run typecheck
• bun test
• bun run lint (warnings only, pre-existing style/size warnings remain)
• bun run format:check
• bun run build:npm

Closes #94