Accept a version argument only if the secret has that version (closes #67)

version rm, version promote and get --version joined the version
argument into a path unchecked, so "", ".", "..", "../../.." removed or
read every version, the secret, the vault or directories above it.

A version is now accepted only if it is one of the versions
ListVersions lists for the secret, compared by name before any path is
built (secret.VersionExists, used by all three). An empty --version is
rejected instead of meaning the current version: GetSecretVersion no
longer treats "" as current, and GetSecret looks the current version
up itself.

Model: opus-5-5

TODO.md

@@ -25,6 +25,13 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-03: `version rm`, `version promote` and `get --version`
+  accept a version only if it is one of the versions `version list`
+  lists for that secret, compared as typed before any path is built
+  (`secret.VersionExists`), and touch nothing otherwise. An empty
+  `--version` is rejected instead of meaning the current version.
+  Before, `secret version rm x ../../..` deleted the whole vault,
+  `secret version rm x ..` the secret, and `.` or `""` every version.
 - 2026-10-03: Key material is wiped on every exit: `Entry()` returns
   the exit code after its deferred `memguard.Purge()` has run, and only
   `main` calls `os.Exit`. SIGINT and SIGTERM go through memguard's