Multiple Windows

I have been using the new Github Copilot app for a few months now. It is really growing on me. One of the biggest limitations I see is the inability to have multiple windows open at once. I know that I can have multiple sessions running in the one window, but I do like the ability when using VS Code or Github Copilot CLI I can have multiple windows open if I'm working on multiple projects.

The issue names no files, tests, or entry points. Start by locating how the desktop app creates and manages windows and sessions, then clarify the expected behavior for separate projects. Done should support multiple independent app windows, but the issue does not define the required interactions or acceptance tests.

索引モデルが issue の本文から書いたものです。