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/path_traversal_test.go

@@ -306,8 +306,7 @@ func TestInvalidVersionLeavesVaultsUnchanged(t *testing.T) {
 
 // TestInvalidVaultNameLeavesStateUnchanged is a regression test for
 // https://git.eeqj.de/sneak/secret/issues/68, where
-// `secret vault import ..` wrote a long-term key and an unlocker into the
-// state directory itself, and `secret vault select ..` made it the current
+// `secret vault select ..` made the state directory itself the current
 // vault. Each command that takes a vault name must reject an invalid one
 // before building a path from it. The instance is given the mnemonic and
 // the passphrase, and moves and removals use --force, so that only the name
@@ -331,9 +330,6 @@ func TestInvalidVaultNameLeavesStateUnchanged(t *testing.T) {
 		{"vault create %q", func(c *cli.Instance, name string) error {
 			return c.CreateVault(cmd, name)
 		}},
-		{"vault import %q", func(c *cli.Instance, name string) error {
-			return c.VaultImport(cmd, name)
-		}},
 		{"vault select %q", func(c *cli.Instance, name string) error {
 			return c.SelectVault(cmd, name)
 		}},