Ask for the passphrase on the controlling terminal, not stdin (closes #144)

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

TODO.md

@@ -18,6 +18,16 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-14: The passphrase and mnemonic prompts ask on the controlling
+  terminal (`/dev/tty`) instead of stdin, so `echo v | secret add s1 --force`,
+  `cat file | secret encrypt key` and `cat file.age | secret decrypt key` ask
+  for the passphrase at a terminal instead of failing
+  (https://git.eeqj.de/sneak/secret/issues/144). Only a process with no
+  controlling terminal gets the error that names the variable to set. On SIGINT
+  or SIGTERM, `Entry()` restores the settings of that terminal, not of stdin,
+  since that is where the prompt turns echo off. `secret init` and
+  `secret vault create` no longer write a newline to stderr after the mnemonic
+  is typed, since the prompt ends that line on the terminal.
 - 2026-10-14: `secret generate mnemonic`, `secret vault list` and
   `secret unlocker list` print their output to stdout, as `secret list` does, so
   it can be piped or captured (https://git.eeqj.de/sneak/secret/issues/138).