Ability to set location of skills and mcp servers

### Feature summary

Button to set location of skills and mcp servers

### What problem are you trying to solve?

You can't set or change the location of global skills and MCP servers within the app, like you can for default location for repos. Currently it detects skills/MCP from C:\Users\\.copilot\ path (in windows).

Currently im duplicating skills in two directories to ensure the app picks it up.

### Proposed solution

A button to enable users to set the default location of skills, MCP servers etc.

### Workflow impact

All users affected.

Enable users to customise default location of more GitHub copilot configurations according to their preferences.

### Installation context

_No response_

### Additional context

_No response_

No files, tests, or entry points are named in the issue. Start by locating the existing repository default-location setting and the code that discovers global skills and MCP servers from the Windows .copilot path; done means users can configure the location in the app and discovery works without duplicated skills.

索引モデルが issue の本文から書いたものです。