calendar: add --alarm flag to send command for custom reminder triggers

Previously the send command hardcoded a 1-day VALARM. Now accepts
--alarm with duration format (e.g. 1h, 30m, 2d), defaulting to 1d.

skills/calendar/TESTING.md

@@ -18,8 +18,10 @@ Generates the ICS and MIME email without sending. Check that:
 - MIME has `Content-Type: text/calendar; method=REQUEST`
 - Only `--to` recipients appear as attendees
 - Times and timezone look correct
+- ICS has `BEGIN:VALARM` with correct `TRIGGER` duration
 
 ```bash
+# Default alarm (1 day before)
 $SKILL_DIR/scripts/calendar.sh send \
   --to "mail@luyx.org" \
   --subject "Test Invite" \
@@ -27,6 +29,16 @@ $SKILL_DIR/scripts/calendar.sh send \
   --start "${TEST_DATE}T15:00:00" \
   --end "${TEST_DATE}T16:00:00" \
   --dry-run
+
+# Custom alarm (1 hour before)
+$SKILL_DIR/scripts/calendar.sh send \
+  --to "mail@luyx.org" \
+  --subject "Test Invite (1h alarm)" \
+  --summary "Test Event (1h alarm)" \
+  --start "${TEST_DATE}T15:00:00" \
+  --end "${TEST_DATE}T16:00:00" \
+  --alarm 1h \
+  --dry-run
 ```
 
 ## 2. Live Send: Self-Invite