chore: delete obsolete documentation

- Delete archived docs directory (docs/archived/)
- Remove obsolete phase planning documents:
  - docs/features/PHASE1.5_DDL_IF_EXISTS_DESIGN.md
  - docs/features/PHASE7_JOIN_COLLATIONS.md
  - docs/features/PHASE9_LOCALE_COLLATIONS_DESIGN.md
  - docs/collation/PHASE_IMPLEMENTATION.md
  - docs/graphrag/PHASE4_DESIGN.md
  - docs/graphrag/PHASE5_1_FEATURE_GUIDE.md
  - docs/graphrag/PHASE6_DESIGN.md
  - docs/graphrag/PHASE6_2_SUMMARY.md
  - docs/graphrag/PHASE6_3_DESIGN.md
- Remove EVENT_SOURCING_RFC.md (superseded by SharpCoreDB.EventSourcing package)
- Remove SQLITE_POSTGRESQL_COMPATIBILITY_TODO.md (superseded by current compatibility docs)
- Update broken references in docs/features/README.md, docs/migration/README.md,
  docs/server/README.md, docs/graphrag/00_START_HERE.md, docs/collation/COLLATION_GUIDE.md,
  and docs/graphrag/CUSTOM_HEURISTICS_GUIDE.md to point to current documentation

These are obsolete phase-status, kickoff, completion, and superseded planning
documents that have been replaced by current implementation and maintained guidance.

Author: MPCoreDeveloper

docs/collation/COLLATION_GUIDE.md

@@ -661,7 +661,6 @@ SELECT 'Café' = 'cafe' COLLATE UNICODE;  -- TRUE (full normalization)
 
 ## See Also
 
-- [Phase 7: JOIN Collations](../features/PHASE7_JOIN_COLLATIONS.md)
 - [EF Core Integration](../EFCORE_COLLATE_COMPLETE.md)
 - [Collation Performance Tests](../benchmarks/Phase5_CollationQueryPerformanceBenchmark.cs)