refactor: unify get_{luks,zfs}_passphrase and get_root_password

Extract the prompt/confirm/min-length loop into prompt_password() in
lib/common.sh using a nameref for the output variable, so UI output
stays on the terminal (no command-substitution capture) and the three
callers collapse from ~30 lines each to a single helper call.

- get_luks_passphrase() — min 8 chars
- get_zfs_passphrase()  — min 8 chars
- get_root_password()   — no min (was unchecked before; preserved)

5 bats tests added: match+min-ok path, length-retry loop,
mismatch-retry loop, min_len=0 disables check, empty passphrase
when min_len=0. make test: 58/58.

0 files changed, 0 insertions, 0 deletions