Add return 0 as the last statement in ~46 shell functions across
17 files that previously relied on implicit return codes
(SonarCloud rule shelldre:S7682).
Replace sesh-gum.sh and sesh-tmux.fish with a unified sesh.sh that
cascades through available tools: gum, fzf-tmux, fzf, bash select.
Falls back to native tmux choose-tree if sesh is not installed.
