feat: scaffold MyPassCore Swift Package

- Add MyPassCore local Swift Package with directory structure for
  Models, KDBX, TOTP, Keychain, AutoFill, and Session modules
- Add KeePassKit as git submodule (Vendor/KeePassKit) with SPM wrapper
  Package.swift since upstream has no native SPM support
- Delete SwiftData boilerplate Item.swift from main app target

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

MyPass/Item.swift

@@ -1,18 +0,0 @@
-//
-//  Item.swift
-//  MyPass
-//
-//  Created by Christophe Vila on 21/05/2026.
-//
-
-import Foundation
-import SwiftData
-
-@Model
-final class Item {
-    var timestamp: Date
-    
-    init(timestamp: Date) {
-        self.timestamp = timestamp
-    }
-}