Feature Request: Auto-import skills and agents from user's .github repo

## Summary

Allow Copilot CLI to automatically discover and load skills and agents from a user's `/.github` repository on GitHub, similar to how `.github` repos already serve as a convention for profile READMEs and org-wide configurations.

## Problem

Today, personal skills and agents must be manually placed in `~/.copilot/skills/` and `~/.copilot/agents/`. This means:

- Skills/agents aren't synced across machines — if I switch between a laptop, desktop, and codespace, I have to manually copy files to each environment.
- There's no easy way to share a curated set of skills/agents with teammates ("just clone my `.github` repo").
- New machine setup requires extra steps beyond just installing Copilot CLI.

## Proposed Solution

**Option A — Automatic sync:**
If the authenticated user has a `.github` repository (e.g. `sandeeprawat/.github`), Copilot CLI automatically fetches (or git-clones/pulls) it and loads:
- `skills/` → as personal skills
- `agents/` → as personal agents

This would happen at CLI startup (with caching so it doesn't slow things down on every launch). A config flag could control the behavior: `"importGitHubHomeRepo": true | false`.

**Option B — Manual command:**
A new slash command like `/skills import-github` or `/import-dotgithub` that fetches the user's `.github` repo on demand and registers the skills/agents found in it.

**Ideally both** — auto-sync by default with a manual command for on-demand refresh.

## Benefits

- **Zero-config portability** — Skills and agents follow the user across machines, codespaces, and environments automatically.
- **Leverages existing GitHub convention** — The `/.github` repo is already a well-known pattern.
- **Shareability** — Users can point colleagues to their `.github` repo as a curated skill/agent collection.
- **Org-level extension** — Could naturally extend to org `.github` repos (`.github-private`) for org-wide skills, complementing the existing org-level agent support.

## Open Questions

- Should this be opt-in or opt-out?
- How to handle conflicts between repo-fetched skills and locally defined skills with the same name?
- Caching/refresh strategy — pull on every startup vs. periodic vs. manual only?
- Security: should there be a trust prompt before loading remotely-fetched skills for the first time?