[New Plugin] dotnet-signalr — skills for ASP.NET Core SignalR

## Proposed Plugin: dotnet-signalr

SignalR is a widely used ASP.NET Core library but there are no
skills covering it in this repo yet.

## Proposed Skills

- **signalr-hub-setup** — setting up a SignalR hub with ASP.NET Core,
DI registration, mapping, and common configuration
- **signalr-redis-backplane** — scaling SignalR across multiple server
instances using Redis
- **signalr-rate-limiting** — applying rate limiting to SignalR hub
endpoints using ASP.NET built-in rate limiter
- **signalr-connection-management** — managing connection lifecycle,
OnConnectedAsync, OnDisconnectedAsync, room/group patterns

## Why

These are non-obvious areas where developers frequently run into
issues that a well-written skill would prevent.

Happy to start with one skill and iterate from there.