Add selectable Fish/Zellij shell and session backends

[Pyrobrick]

Summary

• Add optional shell (fish, zsh, or bash) and session_backend (zellij or tmux) selectors.
• Preserve the existing Zsh/Bash and tmux behavior when the new selectors are omitted, so Supervisor's merged defaults do not silently migrate existing installations.
• Keep the root account on Bash for OpenSSH command compatibility and dispatch only interactive logins through the selected shell/session wrapper.
• Preserve shell login semantics when Zellij or tmux starts the configured shell.
• Add backend regression tests, a dedicated CI job, and updated user documentation.
• Use the Fish and Zellij package versions provided by the Alpine 3.24 base image.

Compatibility behavior

• Existing configurations keep using the legacy zsh option and tmux.
• Direct SSH commands such as ssh host "ha core info" run through Bash without entering a multiplexer.
• Non-root SSH users retain the configured root handoff for both interactive and command-mode sessions.
• Fish and Zellij are opt-in until an explicit migration mechanism can distinguish fresh installs from upgraded Supervisor configuration.
• Mosh bootstraps through command-mode SSH and therefore starts Bash instead of the selected interactive shell or shared-session backend.

Validation

Local checks completed:

• ssh/tests/test-terminal-backends.sh
  • legacy and explicit selector precedence
  • Fish/Zsh/Bash path selection
  • Zellij/tmux dispatch
  • direct-command quoting and exit-status propagation
  • non-interactive multiplexer bypass
  • root account shell rewrite
  • Bash and POSIX shell syntax checks

The dedicated regression job runs alongside the reusable app CI. The reusable workflow already lints the app and builds both configured architectures (amd64 and aarch64).

Remaining integration checks

• Confirm the GitHub Actions jobs run successfully on this fork.
• Smoke-test ttyd, interactive SSH, direct SSH commands, and shared sessions in a Home Assistant add-on runtime.

This remains a draft until those integration checks pass.