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-packets.md

@@ -0,0 +1,44 @@
+---
+name: migrate-packets
+description: >
+  Trigger: when converting RunUO Packet subclasses, PacketWriter/PacketReader, or packet handler registration to ModernUO span-based packets.
+  Covers: outgoing packet conversion, incoming handler conversion, SpanWriter/SpanReader.
+---
+
+# RunUO -> ModernUO Packet Migration
+
+## When This Activates
+- Converting `Packet` subclasses to static create methods
+- Converting `PacketHandlers.Register()` to `IncomingPackets.Register()`
+- Converting `PacketWriter`/`PacketReader` to `SpanWriter`/`SpanReader`
+
+## Conversion Steps (Outgoing)
+1. Create static class: `public static class OutgoingMyPackets`
+2. Define length constant
+3. Convert constructor to `static void CreateXxx(Span<byte> buffer, ...)`
+4. Replace `m_Stream.Write(x)` with `writer.Write(x)`
+5. Create `SendXxx(this NetState ns, ...)` extension method
+6. Check `ns.CannotSendPackets()` at start
+7. Use `stackalloc byte[length].InitializePacket()` for buffer
+8. Replace `ns.Send(new Packet(...))` with `ns.SendXxx(...)`
+
+## Conversion Steps (Incoming)
+1. Change `PacketHandlers.Register(id, len, ingame, new OnPacketReceive(H))` to `IncomingPackets.Register(id, len, ingame, &H)` in `unsafe Configure()`
+2. Change handler: `void H(NetState, PacketReader)` -> `void H(NetState, SpanReader)`
+3. Replace read calls: `pvSrc.ReadString()` -> `reader.ReadAsciiSafe()`
+
+## Quick Mapping
+| RunUO | ModernUO |
+|---|---|
+| `class X : Packet` | Static `CreateX(Span<byte>)` method |
+| `m_Stream.Write(val)` | `writer.Write(val)` (SpanWriter) |
+| `PacketWriter` | `SpanWriter` |
+| `PacketReader` / `pvSrc` | `SpanReader` / `reader` |
+| `ns.Send(new X(...))` | `ns.SendX(...)` extension |
+| `PacketHandlers.Register(...)` | `IncomingPackets.Register(... &handler)` |
+| `new OnPacketReceive(H)` | `&H` (function pointer) |
+
+## See Also
+- `dev-docs/runuo-migration-docs/05-packets-networking.md` -- detailed migration reference
+- `dev-docs/networking-packets.md` -- complete ModernUO networking system
+- `dev-docs/claude-skills/modernuo-networking.md` -- ModernUO networking skill