calendar: add migration guide from reminder_check.py to VTODO

skills/calendar/MIGRATION.md

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+# Migration: reminder_check.py → calendar todo
+
+## What's changing
+
+| Before | After |
+|--------|-------|
+| `scripts/reminder_check.py` | `skills/calendar/scripts/calendar.sh todo` |
+| `reminders/active.md` (markdown table) | `calendars/tasks/*.ics` (RFC 5545 VTODO) |
+| Local only, no sync | CalDAV sync to all devices via vdirsyncer |
+| No native reminders | VALARM triggers on phone/desktop |
+| Cron reads markdown | Cron reads .ics files |
+
+## Prerequisites
+
+- [ ] `calendar.sh todo add --dry-run` works (tested in current session)
+- [ ] `~/.openclaw/workspace/calendars/tasks/` directory exists (auto-created by `todo add`)
+- [ ] vdirsyncer has a `cal/tasks` pair configured
+- [ ] Live tests 8-12 from `TESTING.md` pass on the Linux machine
+
+## Step 1: Migrate pending reminders
+
+Two items are pending in `reminders/active.md`:
+
+```bash
+SKILL_DIR=~/.openclaw/workspace/skills/calendar
+
+$SKILL_DIR/scripts/calendar.sh todo add \
+  --summary "跟进iui保险报销" \
+  --due "2026-03-25" \
+  --priority medium \
+  --description "确认iui（人工授精）费用保险报销进度，避免过期"
+
+$SKILL_DIR/scripts/calendar.sh todo add \
+  --summary "打电话给progyny问iui报销" \
+  --due "2026-04-04" \
+  --priority medium \
+  --description "询问iui报销相关事宜"
+```
+
+Verify:
+```bash
+$SKILL_DIR/scripts/calendar.sh todo list
+```
+
+## Step 2: Set up cron
+
+Ask the agent to set up a daily cron job via `openclaw cron`:
+
+> Set up a daily cron at 8:00 AM PST that runs:
+> `~/.openclaw/workspace/skills/calendar/scripts/calendar.sh todo check`
+> and emails the output to mail@luyx.org with subject "📋 今日待办清单"
+
+The old `reminder_check.py` cron in `crontab.txt` should be removed by the agent at the same time.
+
+## Step 3: Verify
+
+- [ ] `todo list` shows both migrated items
+- [ ] `vdirsyncer sync` completes without errors
+- [ ] Todos appear on phone/CalDAV client
+- [ ] Next morning's cron email arrives with the new format
+
+## Step 4: Clean up (optional, after confidence period)
+
+After a few days of successful operation:
+
+- `reminders/active.md` — can be archived or deleted
+- `scripts/reminder_check.py` — can be removed
+- `reminders/` directory — can be removed
+- Old cron entry in `crontab.txt` — should already be gone
+
+## Rollback
+
+If something goes wrong, the old system is still intact:
+- `reminders/active.md` is unchanged
+- `scripts/reminder_check.py` still works
+- Re-add the old cron entry to restore the previous behavior