The passphrase and mnemonic prompts open /dev/tty and ask there, so
`echo v | secret add s1 --force`, `secret encrypt` and `secret decrypt`
with piped input ask at a terminal instead of failing. Only a process
with no controlling terminal still gets the error naming the variable to
set. `secret init` and `secret vault create` no longer write a newline
to stderr after the mnemonic.
On SIGINT or SIGTERM, Entry() restores the settings of /dev/tty instead
of stdin, since that is where the prompt turns echo off.
The two tests that pin the no-terminal message run the binary in a new
session, so it has no terminal wherever the tests run.
Unverified by a test: the echo restore after Ctrl-C, checked by hand.
Model: opus-5-5
