feat(backend): implement Phase 1 of Intelligence Layer Migration

- Add SQLite-based persistent memory storage (persistent.rs)
- Create memory persistence Tauri commands (memory_commands.rs)
- Add sqlx dependency to Cargo.toml for SQLite support
- Update memory module to export new persistent types
- Register memory commands in Tauri invoke handler
- Add comprehensive migration plan document

Phase 1 delivers:
- PersistentMemory struct with SQLite storage
- MemoryStoreState for Tauri state management
- 10 memory commands: init, store, get, search, delete,
  delete_all, stats, export, import, db_path
- Full-text search capability
- Cross-session memory retention

Reference: docs/plans/INTELLIGENCE-LAYER-MIGRATION.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src-tauri/src/memory/mod.rs

@@ -8,10 +8,12 @@
 
 pub mod extractor;
 pub mod context_builder;
+pub mod persistent;
 
 // Re-export main types for convenience
-// Note: Some types are reserved for future memory integration features
-#[allow(unused_imports)]
 pub use extractor::{SessionExtractor, ExtractedMemory, ExtractionConfig};
-#[allow(unused_imports)]
 pub use context_builder::{ContextBuilder, EnhancedContext, ContextLevel};
+pub use persistent::{
+    PersistentMemory, PersistentMemoryStore, MemorySearchQuery, MemoryStats,
+    generate_memory_id,
+};