Need to be able to edit local files directly without creating worktrees, huge blocker with massive monorepos

Copilot times out making worktrees, massive monorepos take up hundreds of gigabytes of space, build artifacts even more. Why can't it just edit the local repo directly?

You can't even force it to use a worktree you created yourself:

```Not from this chat. I can inspect `C:\copilot-worktrees\1JS-worktree`, but it is not registered as a writable app project session, and general chat cannot edit repository folders directly.```

The issue names no files, tests, or entry points. Start by tracing how writable app project sessions and Copilot worktrees are registered, then determine the supported behavior for a user-created local repository. Done means a massive local repository can be edited without creating a managed worktree or timing out.

索引モデルが issue の本文から書いたものです。