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-systems.md

@@ -0,0 +1,45 @@
+---
+name: migrate-systems
+description: >
+  Trigger: when converting multi-file RunUO engines or systems (crafting, spawners, economy, quests) to ModernUO.
+  Covers: system mapping, conversion order, file organization, cross-reference handling.
+---
+
+# RunUO -> ModernUO Multi-File System Migration
+
+## When This Activates
+- Converting RunUO systems with multiple interdependent files
+- Converting custom engines (crafting, spawners, economy, quests)
+- Organizing RunUO `Scripts/Custom/` code into ModernUO structure
+
+## Conversion Order
+1. **Data types / enums** -- Just naming and namespace changes
+2. **Persistence classes** -- `EventSink.WorldSave` -> `GenericPersistence`
+3. **Core entities (Items/Mobiles)** -- Full serialization migration
+4. **Gumps** -- Convert to `DynamicGump`/`StaticGump`
+5. **Commands** -- Usually minimal changes
+6. **Packets** -- Convert to `SpanWriter` if custom packets exist
+7. **Entry point** -- Update Configure/Initialize registration
+
+## File Organization
+| RunUO | ModernUO |
+|---|---|
+| `Scripts/Custom/MySystem/` | `Projects/UOContent/Engines/MySystem/` or `Projects/UOContent/Systems/MySystem/` |
+| `Scripts/Items/X.cs` | `Projects/UOContent/Items/{Category}/X.cs` |
+| `Scripts/Mobiles/X.cs` | `Projects/UOContent/Mobiles/{Category}/X.cs` |
+| `Scripts/Gumps/X.cs` | `Projects/UOContent/Gumps/X.cs` |
+
+## Configuration Migration
+| RunUO | ModernUO |
+|---|---|
+| XML config files | `ServerConfiguration.GetOrUpdateSetting()` or `JsonConfig` |
+| Custom .cfg parsing | `ServerConfiguration` for simple, `JsonConfig` for complex |
+
+## Testing
+After converting: `dotnet build`, fix errors, test [add for items, verify gumps, check persistence across save/restart.
+
+## See Also
+- `dev-docs/runuo-migration-docs/10-systems-engines.md` -- detailed system migration patterns
+- `dev-docs/configuration.md` -- ModernUO configuration system
+- `dev-docs/claude-skills/modernuo-configuration.md` -- ModernUO configuration skill
+- All other migrate-* skills for system-specific guidance