feat(cron): add one-time 'at' schedule to skill docs and show timezone in system prompt

nanobot/skills/cron/SKILL.md

@@ -7,10 +7,11 @@ description: Schedule reminders and recurring tasks.
 
 Use the `cron` tool to schedule reminders or recurring tasks.
 
-## Two Modes
+## Three Modes
 
 1. **Reminder** - message is sent directly to user
 2. **Task** - message is a task description, agent executes and sends result
+3. **One-time** - runs once at a specific time, then auto-deletes
 
 ## Examples
 
@@ -24,6 +25,11 @@ Dynamic task (agent executes each time):
 cron(action="add", message="Check HKUDS/nanobot GitHub stars and report", every_seconds=600)
 ```
 
+One-time scheduled task (compute ISO datetime from current time):
+```
+cron(action="add", message="Remind me about the meeting", at="<ISO datetime>")
+```
+
 List/remove:
 ```
 cron(action="list")
@@ -38,3 +44,4 @@ cron(action="remove", job_id="abc123")
 | every hour | every_seconds: 3600 |
 | every day at 8am | cron_expr: "0 8 * * *" |
 | weekdays at 5pm | cron_expr: "0 17 * * 1-5" |
+| at a specific time | at: ISO datetime string (compute from current time) |