Add vacation mode

[JulianSarkinovic]

Summary

Adds vacation mode: a way to ground toad without uninstalling it. While on vacation:

• No passive monitoring, no digest analysis (Toad King), no PR review/CI fix polling. No agent or LLM calls happen; the digest spawn path is also hard-guarded.
• Direct interactions (an @mention, keyword or emoji trigger, CTA button, or tadpole-request reaction) get a fixed decline reply, and the message is saved to a new vacation_messages table for review when toad returns.
• Everything else is dropped before any processing.

Toggling from Slack (no restart needed)

• A mention containing "go on vacation" or "vacation time" starts the vacation (toad replies with a send-off rhyme).
• A mention containing "come back" (or "back from vacation", "vacation is over") ends it.
• The state persists in the settings table and survives restarts.
• vacation_admins (Slack user IDs) restricts who can toggle; empty = anyone.
• vacation_mode: true in config (or TOAD_VACATION_MODE=1) forces vacation on and locks out the Slack wake-up.

Not gated: the MCP server and toad run CLI (operator-controlled), and repo sync / worktree cleanup housekeeping.

Rollout

Merging changes nothing by itself. After release, once the daemon is on the new version (auto-update or toad update + toad restart), mention toad in Slack with "go on vacation".