Add Heartbeat

Adds a periodic heartbeat that wakes the agent on a cron schedule, runs through a checklist of cheap checks, and only messages the user if something needs attention.

What it does

1. Creates groups/main/HEARTBEAT.md with a template checklist
2. Registers a scheduled task that runs the heartbeat on a cron (default: every 30 minutes)
3. The agent reads the checklist, runs each check, and stays silent if everything is fine

Setup

1. Create HEARTBEAT.md (if not exists)

Check if groups/main/HEARTBEAT.md exists. If not, create it with this template:

# Heartbeat Checks