Write encrypt and decrypt output only once they succeed (closes #143)

`secret encrypt` and `secret decrypt` emptied the file named by
`--output` before reading any input, so a failure left it empty, and a
file given as both `--input` and `--output` was emptied before it was
read. They now write into a temporary file of mode 0600 next to it and
rename that over it only on success, using the new
`secret.WriteFileAtomicFunc`, which `secret.WriteFileAtomic` now calls.
A device or pipe given as `--output`, such as `/dev/null`, is written to
directly instead of being replaced by a file.

Judgement call: the device and pipe case goes beyond the plan.
Judgement call: a symbolic link given as `--output` is replaced, not followed.

Model: opus-5-5

TODO.md

@@ -18,6 +18,15 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-14: A failed `secret encrypt` or `secret decrypt` leaves the file
+  named by `--output` as it was, and the same file can be given as `--input` and
+  `--output` (https://git.eeqj.de/sneak/secret/issues/143). Both emptied the
+  output file before reading any input, so a wrong key left it empty, and a file
+  given as both was emptied before it was read. They now write the result into a
+  temporary file next to it and rename that over it once they have succeeded,
+  with the new `secret.WriteFileAtomicFunc`, which `secret.WriteFileAtomic` now
+  calls; the output file has mode 0600. A device or a pipe given as `--output`,
+  such as `/dev/null`, is still written to directly.
 - 2026-10-12: A mnemonic typed with extra spaces, tabs or line breaks gives the
   same long-term key as its words separated by single spaces
   (https://git.eeqj.de/sneak/secret/issues/135). The key was derived from the