Add a Flagsmith skill to skills.sh

[Skills.sh](https://skills.sh/) is one of the largest directory of Skills for AI agents, managed by the Vercel team.

To increase our visibility within the AI-assisted SDLC, we should publish one or a few Flagsmith skills that will help AI agent navigate our MCP server and provide additional context as to when and how to you the MCP in different stages of the flag lifecycle.