calendar: add contacts system to prevent recipient address hallucination

Agent hallucinated "xiaojuzi@meta.com" instead of looking up the correct
address from USER.md. Prompting rules can't reliably prevent this, so
recipient validation is now enforced at the tool level.

- Add contact list/add/delete subcommands (vCard .vcf files synced via CardDAV)
- send --to now resolves through contacts: accepts names, name:type, or
  known emails; rejects unknown addresses with available contacts shown
- Multi-email contacts supported with type qualifier (e.g. "小橘子:work")
- Adding contacts and sending are separate operations (no chaining)

skills/calendar/scripts/calendar.sh

@@ -4,9 +4,12 @@
 # Usage:
 #   ./calendar.sh send [options]          # send a calendar invite (supports --rrule)
 #   ./calendar.sh reply [options]         # accept/decline/tentative
-#   ./calendar.sh event list [options]    # list/search calendar events
-#   ./calendar.sh event delete [options]  # delete an event by UID or summary
-#   ./calendar.sh todo add [options]      # create a todo
+#   ./calendar.sh event list [options]      # list/search calendar events
+#   ./calendar.sh event delete [options]    # delete an event by UID or summary
+#   ./calendar.sh contact list              # list all contacts
+#   ./calendar.sh contact add [options]     # add a contact
+#   ./calendar.sh contact delete [options]  # delete a contact
+#   ./calendar.sh todo add [options]        # create a todo
 #   ./calendar.sh todo list [options]     # list pending todos
 #   ./calendar.sh todo edit [options]     # edit a todo's fields
 #   ./calendar.sh todo complete [options] # mark todo as done