Skip corrupt unlocker metadata in unlocker select and remove (closes #72)

findUnlockerByID failed on the first unlocker directory whose metadata
could not be checked, read or parsed, so `secret unlocker select` and
`remove` failed when one sorted before the unlocker asked for. It now
skips such a directory with the warning ListUnlockers gives, through
the code both now share. A skipped directory is removed by its
directory name, with RemoveDirAtomic, and cannot be selected. Removing
one whose metadata file is missing or corrupt never counts as removing
the last unlocker; removing one whose metadata file cannot be checked
for or read does, since it may be the only working unlocker.

Model: opus-5-5

TODO.md

@@ -25,6 +25,16 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-04: `secret unlocker select` and `secret unlocker remove`
+  skip, with the warning `unlocker list` gives, an unlocker directory
+  whose metadata file cannot be checked for, read or parsed, instead of
+  failing when it sorts before the unlocker asked for. Such a directory,
+  or one without a metadata file, is removed by its directory name, the
+  name the warning gives; only the directory is removed, since its type
+  is unknown. Removing one whose metadata file is missing or corrupt
+  never counts as removing the last unlocker. Removing one whose metadata
+  file cannot be checked for or read always does, since it may be the
+  only working unlocker, so in a vault with secrets it needs `--force`.
 - 2026-10-04: A failed command prints its error once, without the usage
   text after it (https://git.eeqj.de/sneak/secret/issues/41). Usage is
   still printed for a command called wrongly: wrong number of arguments,
@@ -121,7 +131,7 @@ Bring the repo into policy compliance in one commit:
       which `vault create` has already made the current vault;
     - from an unlocker add stopped before its metadata is written, a
       directory that `unlocker list` warns about and `unlocker rm`
-      cannot remove;
+      removes only by its directory name;
     - data under a `.tmp-` name in the state directory: a secret or
       version being added, or the secret, version, unlocker or vault
       being removed, encrypted keys included. Nothing deletes it; it