docs: optimize new-pattern.md based on actual workflow

Streamline guide based on developing 4 patterns:
- Segment Tree/Fenwick, Line Sweep, Tree DP, Bitmask DP

Key changes:
- Reduce from complex multi-phase to clear 8-phase workflow
- Update problem count from 8-15 to 3-6 (more practical)
- Add concrete commit structure for each phase
- Include real example patterns with file counts
- Add directory structure example from Bitmask DP
- Simplify prerequisites and deliverables tables
- Remove redundant sections (documentation-only workflow)
- Add merge to main steps in Phase 8

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: lufftw