Address PR #775 review feedback

- Remove AI-generated documentation files (REFACTORING_COMPLETE.md, plans/*.md)
- Update minimum iOS version from 16 to 17 (enables @Observable in future)
- Convert SystemMessagingService to BitchatMessage.system() factory method
  * Removes method overload ambiguity
  * Cleaner API as suggested in review
  * Deleted SystemMessagingService.swift
- Clean up deinit comments (remove unnecessary removeAll() calls)
- Remove conditional OS checks in GeohashBookmarksStore (iOS/macOS only)

All tests passing (23/23)

Package.swift

@@ -6,7 +6,7 @@ let package = Package(
     name: "bitchat",
     defaultLocalization: "en",
     platforms: [
-        .iOS(.v16),
+        .iOS(.v17),
         .macOS(.v13)
     ],
     products: [