Add .gptel file extension support

[La-Loutre]

Automatically enables gptel-mode for files with a .gptel extension. This allows users to create new chat files or reload saved conversations without having to manually enable the mode.

I find this extremely useful. Simply open a new file named myconv.gptel, and you're chatting with the AI in a buffer that can be saved and reopened without running any command. You don't need to run M-x gptel anymore.

[La-Loutre]

Screencast.From.2025-08-09.00-28-36.mp4

[Igrom]

Why is it necessary to define a new mode?

• gptel-mode is supposed to be a minor mode, not a major mode
• you're pinning the mode as text-mode. If opening a new buffer by calling gptel based on the value of gptel-default-mode.

You can achieve the same with a one-liner that respects the value of gptel-default-mode. See master...Igrom:gptel:feature/add-gptel-file-association-to-auto-mode-alist. (though I'm not sure if I'm not abusing auto-mode-alist here).

I'm not even convinced if that's the way to go. File associations could be added to the installation section in the README (e.g. to the :init section of use-package), as it is the case for many other packages.

@karthink FYI (esp. regarding the last point)