The three duplicated PATH blocks in tools/install.sh only warned, so an
unattended install finished with rc=0 and left `mosaic: command not found`
— there was no operator to read the advice and act on it. Measured on a
greenfield Debian 13 sandbox: `--next --yes` installed
@mosaicstack/[email protected] successfully and the CLI was still
unreachable.
Replaces all three copies with one ensure_prefix_on_path helper that
appends the export to ~/.profile (~/.zshenv under zsh) and is a no-op when
the prefix is already on PATH or already in a shell profile.
Not ~/.bashrc: Debian's default .bashrc returns early for non-interactive
shells, so a line appended there is unreachable to `bash -lc`, systemd
units and agent seats — the consumers that need the CLI.
