Add comprehensive Harvest API skill for time tracking, project management,
reporting, and invoice management.

Features:
- Time entry creation, updates, and deletion
- Timer start/stop/restart functionality
- Project and task listing
- Time reports by project and user
- Invoice creation, updates, and client communication
- Expense tracking and reporting
- Client and user management

Uses Harvest REST API v2 with environment-based authentication.
Requires HARVEST_ACCOUNT_ID and HARVEST_ACCESS_TOKEN.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>