Remove secret vault import (closes #139)

`secret vault import` took only a vault without a long-term key, and
`secret init` and `secret vault create` give every vault one, so it
always failed. A mnemonic enters through those two commands, so the
command goes, with its tests and its README entry. The help and the
note of `secret generate mnemonic` named `secret import`, which imports
a file as a secret; they now name `secret init` and
`secret vault create`.

`secret vault import` now prints the help of `secret vault` and exits
0, as cobra does for any subcommand of `secret vault` it does not know.

Judgement call: TestWorkflowMismatch in internal/vault, which replayed
the import step by step, is deleted.

Model: opus-5-5

internal/cli/generate.go

@@ -43,7 +43,7 @@ func newGenerateMnemonicCmd() *cobra.Command {
 		Short: "Generate a random BIP39 mnemonic phrase",
 		Long: `Generate a cryptographically secure random BIP39 ` +
 			`mnemonic phrase that can be used with 'secret init' ` +
-			`or 'secret import'.`,
+			`or 'secret vault create'.`,
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			cli, err := NewCLIInstance()
 			if err != nil {
@@ -115,8 +115,10 @@ func (cli *Instance) GenerateMnemonic(cmd *cobra.Command) error {
 		"   • If you lose this phrase, your secrets cannot be recovered")
 	fmt.Fprintln(os.Stderr, "")
 	fmt.Fprintln(os.Stderr, "Use this mnemonic with:")
-	fmt.Fprintln(os.Stderr, "   secret init    (to initialize a new secret manager)")
-	fmt.Fprintln(os.Stderr, "   secret import  (to import into an existing vault)")
+	fmt.Fprintln(os.Stderr,
+		"   secret init                (to initialize a new secret manager)")
+	fmt.Fprintln(os.Stderr,
+		"   secret vault create <name> (to create another vault)")
 
 	return nil
 }