docs: Consolidate multi-context documentation

Reduce documentation from 1,585 lines to 849 lines (46% reduction) by:
- Eliminating duplicate content (transaction integrity, examples, tables)
- Showing PolicyTypeContextProvider once instead of three times
- Converting full implementation code to pseudocode in design doc
- Consolidating three database compatibility tables into one
- Removing completed checklist items
- Improving structure and navigation

USAGE_GUIDE.md: 603 → 382 lines (37% reduction)
- Clearer quick start section
- One canonical context factory example
- Consolidated transaction behavior section (167 → 80 lines)
- Single database compatibility table
- Removed duplicate PolicyTypeContextProvider from complete example

DESIGN.md: 982 → 467 lines (52% reduction)
- Maintainer-focused technical content
- Pseudocode instead of full implementations (191 → 63 lines)
- Condensed performance section
- Key implementation decisions only
- Removed completed checklist (46 lines)
- Clear cross-references to usage guide

Benefits:
- Zero content duplication
- Clear user vs maintainer separation
- Faster navigation
- Easier maintenance

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

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

Author: thoraj