Print the mnemonic and the vault and unlocker listings to stdout (closes #138)

`secret generate mnemonic`, `secret vault list` and `secret unlocker list`,
in text and `--json` form, printed through cobra's `cmd.Println` and
`cmd.Printf`. Those write to stderr when no output writer is set, and the
binary sets none, so a pipe or `$(...)` got nothing. They now write to
`cmd.OutOrStdout()`, as `secret list` does, and so does "No versions found"
from `secret version list`. Status messages and the warning printed after
the mnemonic stay on stderr.

A new test runs the built binary and reads the stdout of `generate
mnemonic` and of both listings in both forms.

Model: opus-5-5

TODO.md

@@ -18,6 +18,14 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 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).
+  They printed with cobra's `cmd.Println` and `cmd.Printf`, which write to
+  stderr when no output writer is set, so `secret vault list --json | jq`
+  received nothing. "No versions found" from `secret version list` moved to
+  stdout too. Messages such as "Created vault", and the warning after the
+  mnemonic, stay on stderr.
 - 2026-10-14: Removing the current unlocker no longer leaves the vault unable to
   open through the unlockers it still has
   (https://git.eeqj.de/sneak/secret/issues/137). `current-unlocker` went on