Unlock keyring pop-up each time I run neovim

This started a couple of days ago. Every time I run neovim, a pop-up shows up that says:
```
Unlock Keyring
Authentication required
An application wants access to the keyring "Default", but it is locked.
```
Then there is a field for a password (I have no idea what the password for this would be, I don't remember setting this up. There is also a button to `Cancel`.

I just cancel, and everything works just fine. Copilot is able to give to give suggestions, and `:Coplito status` says `Ready`. Its just annoying to have the pop-up every time I open neovim.

The pop-up doesn't show up if I launch nvim with the `--clean` flag, or if I remove the `copilot.vim` repo.

Compare a normal nvim launch with nvim --clean and with copilot.vim removed, then trace the plugin's startup and authentication behavior. Done means launching Neovim no longer opens the Default keyring prompt while Copilot still reports Ready through the status command.

索引モデルが issue の本文から書いたものです。