hack projects --json fails when zellij is missing even if tmux is configured/default

[roodboi]

Context

A new user reported that project lists (including Desktop app population) were not showing up during setup unless zellij was installed.

They were using tmux (/opt/homebrew/bin/tmux) and had it configured as default, but still saw:

hack projects --json fail when zellij was not on PATH
Other commands (hack up/open/logs/doctor) continued to work
After installing zellij, hack projects --json started working immediately.

Repro

Ensure tmux is installed and configured/default mux.
Ensure zellij is not on PATH.
Run:
hack projects --json
Observe failure due to missing zellij executable.
Expected
hack projects --json should work when zellij is missing, and use available mux backends (or gracefully skip unavailable optional backends).

Actual

hack projects --json hard-fails if zellij is missing, even when tmux is available and configured.

Impact

Breaks project discovery/listing workflows
Confusing setup experience for tmux users
Makes optional dependency (zellij) behave like required for project listing

Notes

The root cause is that project/session discovery probes both mux backends and treats missing optional binaries as fatal instead of non-blocking.