Name only the mnemonic when it cannot be read (closes #115)

secret init and secret vault create read the mnemonic with the new
secret.ReadMnemonic, whose every error wraps the new
secret.ErrMnemonicNotRead. It shares the terminal read with ReadPassphrase,
whose errors still wrap ErrPassphraseNotRead. Without a terminal the error
names the environment variable that gives the value instead:
SB_SECRET_MNEMONIC for the mnemonic, SB_UNLOCK_PASSPHRASE for the
passphrase. A test pins the message of init without a terminal.

Model: opus-5-5

TODO.md

@@ -18,6 +18,18 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-04: A mnemonic that cannot be read, in `secret init` and
+  `secret vault create`, gives an error that names the mnemonic only
+  (https://git.eeqj.de/sneak/secret/issues/115). It is read with
+  `secret.ReadMnemonic`, whose every error wraps the new
+  `secret.ErrMnemonicNotRead`; before, it was read with `ReadPassphrase`, so
+  the message said "failed to read mnemonic: failed to read passphrase:" and
+  advised setting `SB_UNLOCK_PASSPHRASE`. Without a terminal it now says
+  "failed to read mnemonic: stdin is not a terminal (piped input or script).
+  Please set the SB_SECRET_MNEMONIC environment variable or run
+  interactively". The passphrase messages no longer repeat "cannot read
+  passphrase" after "failed to read passphrase:", and empty input gives
+  "nothing was entered".
 - 2026-10-04: A failure returns the same error value whichever command hits
   it (https://git.eeqj.de/sneak/secret/issues/113). `internal/cli` no longer
   keeps its own copies of `vault.ErrSecretNotFound`, `ErrVaultNotFound`,