Port abbreviateCwd, abbreviateFolderName, mergeLinear, and
sessionTreePath from web/src/{types,tree}.ts into a maestro-protocol
display module, so the TUI sidebar can group sessions with the same
labels the web UI uses (#117). abbreviate_folder_name returns the
collapsed stem and last segment separately instead of escaped HTML,
so a ratatui caller can style the stem without parsing markup.
maestro-cli's own abbreviate_dir moves into the new module; the CLI
and the worktrees list now call the shared implementation.
Closes#117