Support minimal installations without requiring qterm

Many users are only interested in using `q` as a standalone binary for use with `q chat`. Currently, the documented instructions for installing Q on Linux requires executing the `install.sh` script which is tightly coupled to also installing `qterm` and modifying dotfiles. But, due to #844 many users do not want this pseudoterminal to exist in their shell environment.

The `install.sh` performs a useful function by verifying libc support and automatically copying it to `~/.local/bin`. Therefore, it should make logical sense to update this script by adding a flag (e.g. `install.sh --q-cli`) that will:
1. Verify the system libc version
2. Copy *only* the `q` binary to `~/.local/bin`
3. Manually append `~/.local/bin` to the user's PATH, if it is not already added
4. Go through the login flow

Inizia leggendo install.sh e le istruzioni documentate per l’installazione su Linux, quindi confronta il flusso attuale accoppiato a qterm con i requisiti di questa issue e di #844. Il lavoro è completato quando un’opzione di q-cli verifica libc, installa solo q, garantisce che ~/.local/bin sia incluso in PATH e procede con il login senza installare qterm.

Scritto dal modello di indicizzazione a partire dal testo della issue.