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-serialization.md

@@ -0,0 +1,51 @@
+---
+name: migrate-serialization
+description: >
+  Trigger: when converting RunUO Serialize/Deserialize methods, adding [SerializableField], converting [Constructable] to [Constructible], or migrating manual serialization code.
+  Covers: source-generated serialization, field conversion, version handling, TypeAlias.
+---
+
+# RunUO -> ModernUO Serialization Migration
+
+## When This Activates
+- Converting `Serialize(GenericWriter)`/`Deserialize(GenericReader)` overrides
+- Converting `[Constructable]` to `[Constructible]`
+- Adding `[SerializableField]` attributes
+- Handling save compatibility with `[TypeAlias]`
+
+## Conversion Steps
+1. Add `using ModernUO.Serialization;`
+2. Add `[SerializationGenerator(0, false)]` to class (version 0, false for Item/Mobile)
+3. Add `partial` to class declaration
+4. Convert each serialized field: `private int m_X` -> `[SerializableField(N)] private int _x`
+5. Add `[SerializedCommandProperty(AccessLevel.X)]` if RunUO had `[CommandProperty]`
+6. Add `[InvalidateProperties]` if setter called `InvalidateProperties()`
+7. DELETE the `Serial` constructor
+8. DELETE `Serialize()` and `Deserialize()` overrides
+9. Change `[Constructable]` to `[Constructible]`
+10. Timer fields: leave unserialized, add `[AfterDeserialization]` method
+
+## Quick Mapping
+| RunUO | ModernUO |
+|---|---|
+| `public class Foo : Item` | `[SerializationGenerator(0, false)] public partial class Foo : Item` |
+| `private int m_X` + manual Serialize | `[SerializableField(0)] private int _x` |
+| `[CommandProperty(GM)]` on property | `[SerializedCommandProperty(GM)]` on field |
+| `Foo(Serial serial) : base(serial)` | DELETE |
+| `Serialize(GenericWriter)` | DELETE -- auto-generated |
+| `Deserialize(GenericReader)` | DELETE -- auto-generated |
+| Custom setter with InvalidateProperties() | `[InvalidateProperties]` attribute |
+| Custom setter logic | `[SerializableProperty(N)]` with `this.MarkDirty()` |
+| `reader.ReadMobile()` | `reader.ReadEntity<Mobile>()` |
+| `reader.ReadItem()` | `reader.ReadEntity<Item>()` |
+
+## Anti-Patterns
+- Missing `partial` keyword -> build error
+- Serializing `TimerExecutionToken` -> build error
+- Missing `this.MarkDirty()` in `[SerializableProperty]` setter -> changes not saved
+- Wrong field prefix (`m_` instead of `_`)
+
+## See Also
+- `dev-docs/runuo-migration-docs/02-serialization.md` -- detailed migration reference with before/after
+- `dev-docs/serialization.md` -- complete ModernUO serialization system
+- `dev-docs/claude-skills/modernuo-serialization.md` -- ModernUO serialization skill (patterns, attributes, examples)