Polish remote Herdr shell UX after PR #7 triage

[barbatdev]

Follow-up extracted from stale PR #7.

Keep later: the coherent remote-shell UX leftovers around zsh/zshrc, zsh/scripts/herdr-helpers.zsh, zsh/scripts/ssh-helpers.zsh, zsh/scripts/claude-helpers.zsh, zsh/scripts/worktree-helpers.zsh, herdr/config.toml, ghostty/config, and related docs. Re-evaluate on top of current main; do not resurrect stale cmux/tmux behavior.