fix(axiom-data): stop to-one cascades from deleting parents and siblings

Track.album carried deleteRule: .cascade while Album.tracks cascades too, so
deleting one track deleted its album and, through it, every sibling track.
The NoteTag junction example in swiftdata-migration.md cascaded both of its
to-one sides the same way, and never compiled: Note and Tag had no
initializer.

Drop the cascade from the to-one sides (the default .nullify applies),
declare the junction's inverses, and add the missing initializers.

Verified: deleting one of two tracks now leaves 1 album and 1 track (was 0
and 0); removing one tag from a note keeps every note and tag (was all
deleted). The edited blocks compile against the iOS 27.1 SDK.

.claude-plugin/plugins/axiom/skills/axiom-data/skills/swiftdata-migration.md

@@ -420,10 +420,12 @@ final class NoteTag {
     @Attribute(.unique) var id: String
     var addedAt: Date  // Metadata on relationship
 
-    @Relationship(deleteRule: .cascade)
+    // No cascade on the junction's own sides: removing one tag from one note
+    // would delete that note and that tag, and their cascades would take the rest
+    @Relationship(inverse: \Note.noteTags)
     var note: Note?
 
-    @Relationship(deleteRule: .cascade)
+    @Relationship(inverse: \Tag.noteTags)
     var tag: Tag?
 
     init(id: String, note: Note, tag: Tag, addedAt: Date) {
@@ -445,6 +447,11 @@ final class Note {
     var tags: [Tag] {
         noteTags.compactMap { $0.tag }
     }
+
+    init(id: String, title: String) {
+        self.id = id
+        self.title = title
+    }
 }
 
 @Model
@@ -458,6 +465,11 @@ final class Tag {
     var notes: [Note] {
         noteTags.compactMap { $0.note }
     }
+
+    init(id: String, name: String) {
+        self.id = id
+        self.name = name
+    }
 }
 ```
 

.claude-plugin/plugins/axiom/skills/axiom-data/skills/swiftdata.md

@@ -121,7 +121,9 @@ final class Track {
     @Attribute(.unique) var id: String
     var title: String
 
-    @Relationship(deleteRule: .cascade, inverse: \Album.tracks)
+    // No cascade on this side: deleting one track would delete its album,
+    // and Album.tracks' cascade would then delete every sibling track
+    @Relationship(inverse: \Album.tracks)
     var album: Album?
 
     init(id: String, title: String, album: Album? = nil) {
@@ -805,7 +807,7 @@ Run the sweep on a background context (not the main one), keyed by the natural I
 ```swift
 @Model
 final class Track {
-    @Relationship(deleteRule: .cascade, inverse: \Album.tracks)
+    @Relationship(inverse: \Album.tracks)
     var album: Album?  // ✅ Must be optional for CloudKit
 
     init(album: Album? = nil) {
@@ -887,7 +889,6 @@ let container = try ModelContainer(for: schema, configurations: testConfig)
 @Model
 final class Track {
     @Relationship(
-        deleteRule: .cascade,
         minimumModelCount: 0,
         maximumModelCount: 1,  // Track belongs to at most one album
         inverse: \Album.tracks