calendar: add EXDATE support for cancelling single occurrences of recurring events

event delete on a recurring event previously deleted the entire .ics file,
killing the whole series. Now requires --date (adds EXDATE) or --all (deletes
series) for recurring events, refusing to act without either flag.

skills/calendar/TESTING.md

@@ -382,7 +382,60 @@ $SKILL_DIR/scripts/calendar.sh event list --search "Delete Me"
 - [ ] Other events are untouched
 - [ ] `vdirsyncer sync` ran after delete
 
-## 18. Regression: Existing Invite Commands
+## 18. Event Delete: Cancel Single Occurrence (EXDATE)
+
+Test that `--date` cancels one occurrence of a recurring event without deleting the series.
+
+```bash
+# Create a recurring event (weekly on Saturday, 4 weeks)
+NEXT_SAT=$(python3 -c "from datetime import date,timedelta; d=date.today(); d+=timedelta((5-d.weekday())%7 or 7); print(d)")
+
+$SKILL_DIR/scripts/calendar.sh send \
+  --to "mail@luyx.org" \
+  --subject "EXDATE Test (Sat)" \
+  --summary "EXDATE Test (Sat)" \
+  --start "${NEXT_SAT}T10:00:00" \
+  --end "${NEXT_SAT}T11:00:00" \
+  --rrule "FREQ=WEEKLY;COUNT=4;BYDAY=SA"
+
+# Verify it exists
+$SKILL_DIR/scripts/calendar.sh event list --search "EXDATE Test"
+
+# Cancel just the first occurrence
+$SKILL_DIR/scripts/calendar.sh event delete --match "EXDATE Test" --date "$NEXT_SAT"
+
+# Verify: .ics file still exists (not deleted)
+ls ~/.openclaw/workspace/calendars/home/ | grep -i exdate
+```
+
+**Verify:**
+- [ ] `event delete --match ... --date ...` prints "Cancelled ... (added EXDATE, series continues)"
+- [ ] `.ics` file still exists in calendar dir
+- [ ] `khal list` no longer shows the cancelled date but shows subsequent Saturdays
+
+## 19. Event Delete: Recurring Without --date or --all (Safety Guard)
+
+```bash
+# Try to delete the recurring event without --date or --all — should FAIL
+$SKILL_DIR/scripts/calendar.sh event delete --match "EXDATE Test"
+```
+
+**Verify:**
+- [ ] Script exits with error
+- [ ] Error message explains the two options: `--date` or `--all`
+
+## 20. Event Delete: Recurring With --all
+
+```bash
+# Delete the entire series
+$SKILL_DIR/scripts/calendar.sh event delete --match "EXDATE Test" --all
+```
+
+**Verify:**
+- [ ] .ics file is removed
+- [ ] `event list --search "EXDATE Test"` shows nothing
+
+## 21. Regression: Existing Invite Commands (was #18)
 
 Verify new features didn't break VEVENT flow.
 
@@ -441,3 +494,4 @@ todo list
 | Recurring events on wrong day | DTSTART was not aligned with BYDAY. Delete the event and resend with correct `--start` |
 | SMTP rate limit / EOF error | Too many sends too fast. Wait 10+ seconds between sends (Migadu limit) |
 | Events disappeared after cleanup | **Never use `rm *.ics`** on calendar dirs. Use `event delete --match` instead |
+| Recurring series deleted when cancelling one date | Use `--date YYYY-MM-DD` to add EXDATE, not bare `event delete` (which requires `--all` for recurring) |