Cross-device sync for personal Copilot instructions

Personal instruction files are local to each machine. If I teach Copilot a lesson on one device (e.g. "always write Python scripts to a file instead of passing markdown through PowerShell inline"), that knowledge doesn't carry to my other machines. Since Copilot already authenticates via GitHub, personal instructions could be synced as a GitHub-backed user setting — similar to VS Code Settings Sync. This could be a private gist, a user-scoped API, or a dotfiles-style repo integration. Today the workarounds (symlinks to cloud storage, manually copying files) are fragile and easy to forget.

## Example prompts or workflows

1. I discover that PowerShell mangles backticks and emoji in API calls. I tell Copilot to remember this. On my laptop the next day, Copilot already knows.
2. I set a personal preference like "always use Python 3.12" or "prefer pnpm over npm". It applies on every device I use.
3. I build up a set of personal coding conventions over weeks. When I get a new machine, they're already there after `copilot /login`.
4. A team lead sets org-wide instructions via a repo. A developer supplements with personal preferences that roam with their GitHub account.

## Additional context

This has bitten me across three separate sessions — Copilot keeps re-learning the same PowerShell encoding lesson because session memory is local-only. The session store DB doesn't roam either.

Start by examining how personal instruction files and the local session store DB are located and loaded, then trace the `copilot /login` authentication flow. Define the sync scope, storage approach, privacy and conflict behavior, and how personal settings should be restored on another device before implementation; done means instructions reliably roam between authenticated devices.

Written by the indexing model from the issue text.