feat: add cron tool for scheduling reminders and tasks

nanobot/skills/cron/SKILL.md

@@ -0,0 +1,40 @@
+---
+name: cron
+description: Schedule reminders and recurring tasks.
+---
+
+# Cron
+
+Use the `cron` tool to schedule reminders or recurring tasks.
+
+## Two Modes
+
+1. **Reminder** - message is sent directly to user
+2. **Task** - message is a task description, agent executes and sends result
+
+## Examples
+
+Fixed reminder:
+```
+cron(action="add", message="Time to take a break!", every_seconds=1200)
+```
+
+Dynamic task (agent executes each time):
+```
+cron(action="add", message="Check HKUDS/nanobot GitHub stars and report", every_seconds=600)
+```
+
+List/remove:
+```
+cron(action="list")
+cron(action="remove", job_id="abc123")
+```
+
+## Time Expressions
+
+| User says | Parameters |
+|-----------|------------|
+| every 20 minutes | every_seconds: 1200 |
+| every hour | every_seconds: 3600 |
+| every day at 8am | cron_expr: "0 8 * * *" |
+| weekdays at 5pm | cron_expr: "0 17 * * 1-5" |