docs: Clean up documentation structure

REMOVED (duplicates/superseded):
- INTEGRATION_TESTS.md (superseded by TESTING.md)
- INTEGRATION_TEST_*.md (duplicates of docs/ versions)

ARCHIVED (historical):
- PHASE_3.2_COMPLETE.md → docs/archive/
- PHASE_3.3_COMPLETE.md → docs/archive/
- PHASE_4_CORE_COMPLETE.md → docs/archive/
+ Added docs/archive/README.md to explain archived content

RESULT:
Root now has only essential user-facing docs:
- README.md (plugin overview)
- TESTING.md (testing guide)

All technical/historical docs organized in docs/ hierarchy.

Author: d3xter666