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 gives the warning prune, snapshot
purge and snapshot remove refuse with: both destinations and the two
ways out (revert the configuration, or run vaultik database delete).
Below the table it replaces the local-only note and its prune advice.
With --json it goes to stderr through the logger, so stdout stays the
document, whose local rows show remote_present false.

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.

Model: opus-5-5

internal/vaultik/snapshot_list_test.go

@@ -421,6 +421,74 @@ func TestListSnapshots_LocalOnlyReportedAsDrift(t *testing.T) {
 	assert.NotContains(t, out, "snapshot cleanup")
 }
 
+// TestListSnapshots_BoundElsewhereDoesNotAdvisePrune covers a changed
+// storage_url: the local index is still bound to the old destination,
+// so its records are missing from the new one. prune refuses to run in
+// that state, so the listing must give the choices prune gives instead
+// of advising it.
+func TestListSnapshots_BoundElsewhereDoesNotAdvisePrune(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	const (
+		oldURL = "file:///mnt/backups/old"
+		newURL = "file:///mnt/backups/new"
+	)
+
+	env := newListEnv(t)
+	env.v.Config.StorageURL = newURL
+	require.NoError(t, env.v.Repositories.LocalMeta.Set(context.Background(),
+		database.LocalMetaKeyStorageURL, oldURL))
+	env.addLocal(t, listLocalID, time.Date(2026, 3, 1, 10, 0, 0, 0, time.UTC))
+
+	err := env.v.ListSnapshots(false)
+	require.NoError(t, err)
+
+	out := env.stdout.String()
+
+	assert.Contains(t, out, listLocalID)
+	assert.Contains(t, out,
+		"local index is bound to a different backup destination")
+	assert.Contains(t, out, oldURL)
+	assert.Contains(t, out, newURL)
+	assert.Contains(t, out, "revert storage_url")
+	assert.Contains(t, out, "vaultik database delete")
+	assert.NotContains(t, out, "vaultik prune")
+}
+
+// TestListSnapshots_JSONReportsBoundElsewhere is the --json side of the
+// test above. Every local row comes out with remote_present false, which
+// a script reads as stale local records unless it is told, on stderr,
+// that the local index is bound to a different destination.
+//
+//nolint:paralleltest // captureProcessStderr replaces os.Stderr
+func TestListSnapshots_JSONReportsBoundElsewhere(t *testing.T) {
+	const (
+		oldURL = "file:///mnt/backups/old"
+		newURL = "file:///mnt/backups/new"
+	)
+
+	env := newListEnv(t)
+	env.v.Config.StorageURL = newURL
+	require.NoError(t, env.v.Repositories.LocalMeta.Set(context.Background(),
+		database.LocalMetaKeyStorageURL, oldURL))
+	env.addLocal(t, listLocalID, time.Date(2026, 3, 1, 10, 0, 0, 0, time.UTC))
+
+	stderr := captureProcessStderr(t, func() {
+		require.NoError(t, env.v.ListSnapshots(true))
+	})
+
+	rows := decodeListJSON(t, env.stdout.String())
+	require.Len(t, rows, 1)
+	assert.Equal(t, listLocalID, rows[0].ID)
+
+	assert.Contains(t, stderr, "bound to a different backup destination")
+	assert.Contains(t, stderr, oldURL)
+	assert.Contains(t, stderr, newURL)
+	assert.Contains(t, stderr, "revert storage_url")
+	assert.Contains(t, stderr, "vaultik database delete")
+}
+
 // TestListSnapshots_UnreachableRemoteDegrades covers the promise in the
 // doc comment: an unreachable destination is a warning plus local-only
 // output, never a failure.