docs: consolidate and remove duplicate documentation

- Removed 11 duplicate/redundant documentation files
  * Deleted arrow key fix versions: ARROW_KEY_NAVIGATION_FIX.md, ARROW_KEY_FIX_V2.md
  * Deleted filtering duplicates: DYNAMIC_FILTERING_SUMMARY.md, FILTERING_QUICK_REFERENCE.md
  * Deleted test duplicates: TEST_SUITE_SUMMARY.md, TEST_IMPROVEMENTS_SUMMARY.md
  * Deleted redundant completion doc: REFACTORING_COMPLETE.md
  * Cleaned up root-level temp files: PHASE1_*.md, CODE_QUALITY_*.md, etc.

- Consolidated 9 scattered documents into 3 comprehensive guides
  * Arrow Key: 3 versions → ARROW_KEY_NAVIGATION.md (kept current, removed fixes)
  * Filtering: 3 versions → DYNAMIC_FILTERING.md (merged with keyboard shortcuts, patterns, workflows)
  * Testing: 3 versions → TESTING.md (merged 203+ tests with coverage, fixtures, best practices)

- Reduced documentation file count from 38 to 27 (-29%)
- All content preserved - no information lost
- Verified no broken references or internal links
- Single source of truth for each feature/capability

Benefits:
  ✓ Improved discoverability - users know where to find each topic
  ✓ Easier maintenance - no duplicate content to keep in sync
  ✓ Better organization - clear hierarchy of documentation types
  ✓ Cleaner repository - reduced doc maintenance overhead

🤖 Generated with Claude Code

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

Author: jzhu