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

TODO.md

@@ -18,6 +18,14 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-14: `secret vault import` is gone
+  (https://git.eeqj.de/sneak/secret/issues/139). It 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, which
+  the help and the note of `secret generate mnemonic` now name; they named
+  `secret import`, which imports a file as a secret. `secret vault import` now
+  prints the help of `secret vault`, which cobra prints for any subcommand of
+  `secret vault` that does not exist.
 - 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).