Add cron scheduler and heartbeat runner

[danbao]

Summary

Add two automation features: Cron Scheduler and Heartbeat Runner.

Cron Scheduler

Schedule recurring or one-shot tasks via chat commands:

/cron add "standup" every:24h "summarize yesterday's work"
/cron add "check" */30 * * * * "check for new PRs"
/cron add "reminder" at:2026-04-01T09:00:00 "sprint review today"
/cron list / delete / enable / disable

• Supports cron expressions, fixed intervals (every:5m), and one-shot (at:...)
• Jobs persisted to ~/.weclaw/cron/jobs.json
• Uses robfig/cron/v3 for expression parsing

Heartbeat Runner

Periodic agent check-in driven by ~/.weclaw/HEARTBEAT.md:

• Reads checklist on configurable interval
• Sends to default agent; suppresses HEARTBEAT_OK replies
• 24h dedup to avoid noise
• Configurable active hours and timezone

Config

{
  "heartbeat": {
    "enabled": true,
    "interval": "30m",
    "active_hours": "09:00-18:00",
    "timezone": "Asia/Shanghai",
    "target_user": "wechat_user_id"
  }
}

Changes

• messaging/cron.go — CronScheduler with tick loop
• messaging/cron_store.go — JSON persistence
• messaging/cron_commands.go — /cron chat command parser
• messaging/heartbeat.go — HeartbeatRunner
• config/config.go — HeartbeatConfig type
• cmd/start.go — Start scheduler + heartbeat using first account's client
• messaging/handler.go — /cron routing, exported GetDefaultAgent/GetAgent
• 30+ tests, all passing with -race
• Updated both READMEs