feat: Adds AI skills to migrate from RunUO (#2366)

## Summary

Adds comprehensive RunUO → ModernUO migration documentation and Claude AI skills to help shard owners and script authors convert RunUO 2.7 code to ModernUO.

- **10 migration skills** (`dev-docs/claude-skills/migrate-from-runuo/`) — system-by-system conversion guides (foundation, serialization, timers, gumps, packets, property lists, commands/events, persistence, items/mobiles, systems/engines)
- **12 reference docs** (`dev-docs/runuo-migration-docs/`) — deep-reference with before/after examples, API mapping tables, edge cases, and gotchas
- **Updated existing skills** — `modernuo-timers`, `modernuo-serialization`, and `modernuo-threading` now document that `Serialize()` runs on background threads and timers are not thread-safe
- **Updated `CLAUDE.md`** — added migration skill lookup table

### Key migration patterns covered
- Manual `Serialize()`/`Deserialize()` → source-generated `[SerializableField]`
- `Packet` class hierarchy → static `SpanWriter`/`SpanReader` methods
- `Timer` subclasses → `TimerExecutionToken` fire-and-forget
- `Gump` → `StaticGump<T>`/`DynamicGump` with builders
- `EventSink.WorldSave` → `GenericPersistence`
- `ObjectPropertyList` → `IPropertyList` with string hole rules
- Universal changes: naming (`m_` → `_`), `[Constructable]` → `[Constructible]`, logging, spatial queries

dev-docs/claude-skills/migrate-from-runuo/migrate-persistence.md

@@ -0,0 +1,42 @@
+---
+name: migrate-persistence
+description: >
+  Trigger: when converting RunUO EventSink.WorldSave/WorldLoad manual binary persistence to ModernUO GenericPersistence.
+  Covers: GenericPersistence subclassing, IGenericWriter/IGenericReader, MarkDirty pattern.
+---
+
+# RunUO -> ModernUO Persistence Migration
+
+## When This Activates
+- Converting `EventSink.WorldSave`/`EventSink.WorldLoad` patterns
+- Converting manual `BinaryFileWriter`/`BinaryFileReader` persistence
+- Systems that save custom data outside of Item/Mobile serialization
+
+## Conversion Steps
+1. Create class inheriting `GenericPersistence`: `class MySystem : GenericPersistence`
+2. Add static instance + `Configure()`: `_instance = new MySystem();`
+3. Call `base("SaveName", 10)` in constructor
+4. Move save logic to `override Serialize(IGenericWriter writer)`
+5. Move load logic to `override Deserialize(IGenericReader reader)`
+6. Remove `EventSink.WorldSave`/`WorldLoad` subscriptions
+7. Remove all file management code (Directory.Create, File.Exists, FileStream)
+8. Add `_instance.MarkDirty()` wherever data changes
+9. Replace `writer.Write(mobile.Serial.Value)` -> `writer.Write(mobile)`
+10. Replace `World.FindMobile(reader.ReadInt32())` -> `reader.ReadEntity<Mobile>()`
+
+## Template
+```csharp
+public class MySystem : GenericPersistence
+{
+    private static MySystem _instance;
+    public static void Configure() => _instance = new MySystem();
+    public MySystem() : base("MySystem", 10) { }
+    public override void Serialize(IGenericWriter writer) { /* save */ }
+    public override void Deserialize(IGenericReader reader) { /* load */ }
+}
+```
+
+## See Also
+- `dev-docs/runuo-migration-docs/08-persistence.md` -- detailed migration reference with before/after
+- `dev-docs/serialization.md` -- ModernUO serialization system (IGenericWriter/IGenericReader)
+- `dev-docs/claude-skills/modernuo-serialization.md` -- ModernUO serialization skill