Move scan_from_root, scan_all_roots, get_projects, show_cache_status
and launch_tmux_session from main.rs into a new src/lib.rs, making
them pub so they are testable independently of the binary entrypoint.
main.rs is now a thin entrypoint that imports from tmuxido:: and keeps
only select_project_with_fzf (interactive subprocess, not unit-testable).
Add tempfile = "3" to [dev-dependencies] in preparation for tests.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
