shells: add Ptyxis terminal

[khengyun]

Description

• Adds support for the Ptyxis terminal on Linux.
• Detects the native executable at /usr/bin/ptyxis and opens a new window in the selected repository.
• Works when GitHub Desktop is packaged as a Flatpak and Ptyxis is installed on the host through the existing flatpak-spawn integration.
• Ubuntu 26.04 LTS now provides its default Terminal app through Ptyxis instead of GNOME Terminal.
• Revives shells: add Ptyxis terminal #1131, which was closed after its contributor deleted the repository containing the head branch, and preserves credit for the original implementation.

Screenshots

On Ubuntu 26.04, GitHub Desktop retains an unavailable Open in GNOME Console entry:

Selecting the entry throws Cannot read properties of undefined (reading path) instead of opening a terminal:

Testing

• yarn lint
• yarn compile:dev
• Manually launched Ptyxis 50.1 from the GitHub Desktop Flatpak sandbox with the repository as its working directory.
• yarn test: 1,030 passed and 4 skipped; 2 unrelated existing tests failed locally with Git 2.53.0 because a non-repository directory now returns a fatal error (remote-test.ts and for-each-ref-test.ts).

Release notes

Notes: Added support for the Ptyxis terminal