Fix upcoming Google Calendar event retrieval

[ElliotSlusky]

Summary

• parse Google Calendar all-day events from start.date instead of timestamping them as the current time
• treat generic next/upcoming calendar-event searches as Google Calendar timeline queries and return nearest future events first
• update the research planner guidance and add regression coverage for next calendar event retrieval

Closes #388

Testing

• PYTHONPATH=src python3 -m pytest tests/connectors/test_store.py tests/connectors/test_sync_engine.py tests/connectors/test_gcalendar.py tests/connectors/test_hybrid_search.py tests/agents/test_research_loop.py tests/tools/test_knowledge_search.py
• python3 -m ruff check src/openjarvis/connectors/gcalendar.py src/openjarvis/connectors/hybrid_search.py src/openjarvis/agents/research_loop.py tests/connectors/test_gcalendar.py tests/connectors/test_hybrid_search.py tests/agents/test_research_loop.py
• git diff --check

Note: local full-suite collection is blocked by missing optional dependencies (respx, polars). Local uv 0.5.9 also cannot parse the current uv.lock editable package entry, so the focused suite was run directly with PYTHONPATH=src.