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

internal/cli/unlockers.go

@@ -326,22 +326,25 @@ func (cli *Instance) printUnlockersJSON(
 		return fmt.Errorf("failed to marshal JSON: %w", err)
 	}
 
-	cli.cmd.Println(string(jsonBytes))
+	_, _ = fmt.Fprintln(cli.cmd.OutOrStdout(), string(jsonBytes))
 
 	return nil
 }
 
 // printUnlockersTable prints unlockers in a formatted table
 func (cli *Instance) printUnlockersTable(unlockers []UnlockerInfo) error {
+	out := cli.cmd.OutOrStdout()
+
 	if len(unlockers) == 0 {
-		cli.cmd.Println("No unlockers found in current vault.")
-		cli.cmd.Println("Run 'secret unlocker add passphrase' to create one.")
+		_, _ = fmt.Fprintln(out, "No unlockers found in current vault.")
+		_, _ = fmt.Fprintln(out, "Run 'secret unlocker add passphrase' to create one.")
 
 		return nil
 	}
 
-	cli.cmd.Printf("  %-40s %-12s %-20s %s\n", "UNLOCKER ID", "TYPE", "CREATED", "FLAGS")
-	cli.cmd.Printf("  %-40s %-12s %-20s %s\n",
+	_, _ = fmt.Fprintf(out, "  %-40s %-12s %-20s %s\n",
+		"UNLOCKER ID", "TYPE", "CREATED", "FLAGS")
+	_, _ = fmt.Fprintf(out, "  %-40s %-12s %-20s %s\n",
 		strings.Repeat("-", unlockerIDWidth), strings.Repeat("-", unlockerTypeWidth),
 		strings.Repeat("-", unlockerDateWidth), strings.Repeat("-", unlockerFlagsWidth))
 
@@ -356,7 +359,7 @@ func (cli *Instance) printUnlockersTable(unlockers []UnlockerInfo) error {
 			prefix = "* "
 		}
 
-		cli.cmd.Printf("%s%-40s %-12s %-20s %s\n",
+		_, _ = fmt.Fprintf(out, "%s%-40s %-12s %-20s %s\n",
 			prefix,
 			unlocker.ID,
 			unlocker.Type,
@@ -364,7 +367,7 @@ func (cli *Instance) printUnlockersTable(unlockers []UnlockerInfo) error {
 			flags)
 	}
 
-	cli.cmd.Printf("\nTotal: %d unlocker(s)\n", len(unlockers))
+	_, _ = fmt.Fprintf(out, "\nTotal: %d unlocker(s)\n", len(unlockers))
 
 	return nil
 }