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>

MyPassCore/Package.swift

@@ -0,0 +1,26 @@
+// swift-tools-version: 5.9
+import PackageDescription
+
+let package = Package(
+    name: "MyPassCore",
+    platforms: [.iOS(.v17), .macOS(.v14)],
+    products: [
+        .library(name: "MyPassCore", targets: ["MyPassCore"]),
+    ],
+    dependencies: [
+        .package(path: "../Vendor/KeePassKit"),
+    ],
+    targets: [
+        .target(
+            name: "MyPassCore",
+            dependencies: [
+                .product(name: "KeePassKit", package: "KeePassKit"),
+            ]
+        ),
+        .testTarget(
+            name: "MyPassCoreTests",
+            dependencies: ["MyPassCore"],
+            resources: [.copy("Fixtures")]
+        ),
+    ]
+)