Skip vaultik's own index and PID file in a backup (closes #421)

The default config backs up the home directory, which holds vaultik's local index and PID file, so a backup stored a torn copy of the index it was writing. The scan now skips the index, its SQLite side files and the PID file whatever the config says, including when index_path or its directory is a symlink. database purge removes the same list of index files.

Model: opus-5-5

internal/cli/app.go

@@ -7,11 +7,9 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"path/filepath"
 	"sync"
 	"time"
 
-	"github.com/adrg/xdg"
 	"github.com/spf13/cobra"
 	"go.uber.org/dig"
 	"go.uber.org/fx"
@@ -351,8 +349,7 @@ func runVaultikApp(
 // only one runs at a time; a read-only command runs without it and is not
 // blocked while a mutator holds the lock (opts.Mode).
 func RunWithApp(ctx context.Context, opts AppOptions) error {
-	release, err := acquireLockIfMutating(opts.Mode,
-		filepath.Join(xdg.DataHome, "vaultik"))
+	release, err := acquireLockIfMutating(opts.Mode, pidlock.Dir())
 	if err != nil {
 		return err
 	}