Make snapshot list report a changed destination instead of advising prune (closes #317)

When the local index is bound to a different destination than the
configured one, snapshot list now prints the warning prune, snapshot
purge and snapshot remove refuse with, including its two ways out
(revert the configuration, or run vaultik database delete). It no
longer reports the local records as missing or advises vaultik prune,
which would refuse.

Reading the binding moves out of EnsureStorageBinding into
storageBinding, which records nothing, so the read-only list can call
it without binding a fresh index.

Judgement call: --json output is unchanged; it gives no advice.

Model: opus-5-5

README.md

@@ -323,7 +323,11 @@ read from the manifest.
 
 Snapshots in the local index with no counterpart on the destination
 store are reported below the table as drift, with the `vaultik prune`
-invocation that reconciles them.
+invocation that reconciles them. If the local index is bound to a
+different destination than the configured one (after a change to
+`storage_url`, or `s3.endpoint`, `s3.bucket` or `s3.prefix`), the
+command says so instead and gives the same two ways out that `prune`
+gives: revert the configuration, or run `vaultik database delete`.
 
 If the destination store cannot be listed (unmounted volume,
 permission denied, network down), the command warns, falls back to the