feat: implement RFC-0018 Excel full progress mode (Phase 3)

Add Full progress mode with comprehensive progress tracking:
- TaskStatus enum with icons (✓ ● ○ ⚠) for task state visualization
- Variance column showing schedule deviation (+2d ahead, -3d behind)
- Status column with visual indicators (Complete, InProgress, NotStarted, Behind, Overdue)
- Variance calculation based on expected vs actual progress at status date

Full mode adds 6 progress columns (vs 4 for Columns/Visual modes):
Status, Complete, Remaining, Variance, Act.Start, Act.End

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

Author: 2 people