Add bash tab completion script for pwr CLI commands

Provide a bash completion function for all pwr subcommands and their
flags. The completion script maps each subcommand to its valid arguments:
init completes –server-host, –server-port, –psk, and –local-root;
archive, restore, and ensure complete directory paths; status and list
offer –recursive/-r; log provides –errors/-e and –project; shell
completes bash, zsh, fish, and –init. The completion is installed via
‘source <(pwr completions bash)’ in .bashrc.