Adds CalendarTool implementing the Tool ABC so the ReAct engine can
create, query, update, and delete events autonomously. Resolves
event_type_name and tag_names (look up or create), sets
source="agent" to distinguish agent-created events from manual ones.
- src/agentkit/tools/calendar_tool.py — CalendarTool(Tool)
- tests/unit/tools/test_calendar_tool.py — 13 tests covering all actions