step 2: init command + SQLite storage + migrations + config

- storage.go: DB wrapper, migration runner (in-code SQL strings)
- migrations.go: 001_init (nodes + node_meta + indexes)
- vault.go: Init() creates .verstak/ dirs, config.yml, index.db
- config.go: YAML config read/write
- util/uuid.go: UUIDv7 generator
- cmd/verstak/main.go: init --vault PATH command
- main_test.go: TestInitCreatesVault, TestInitConfigYAML

Acceptance: go build ./... pass, go test ./... pass
Init creates test-vault with .verstak/index.db + config.yml
Repeat Init is safe.

internal/core/vault/vault.go

@@ -0,0 +1,65 @@
+package vault
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"verstak/internal/core/config"
+	"verstak/internal/core/storage"
+	"verstak/internal/core/util"
+)
+
+const currentVaultVersion = 1
+
+// Init creates a vault directory structure at vaultRoot.
+// Calling Init on an already-initialized vault is safe.
+func Init(vaultRoot string) error {
+	vaultRoot = filepath.Clean(vaultRoot)
+
+	dirs := []string{
+		filepath.Join(vaultRoot, ".verstak"),
+		filepath.Join(vaultRoot, ".verstak", "trash"),
+		filepath.Join(vaultRoot, ".verstak", "history"),
+		filepath.Join(vaultRoot, ".verstak", "originals"),
+		filepath.Join(vaultRoot, ".verstak", "thumbnails"),
+		filepath.Join(vaultRoot, ".verstak", "blobs"),
+		filepath.Join(vaultRoot, "spaces"),
+	}
+	for _, d := range dirs {
+		if err := os.MkdirAll(d, 0o750); err != nil {
+			return fmt.Errorf("create %s: %w", d, err)
+		}
+	}
+
+	cfgPath := filepath.Join(vaultRoot, ".verstak", "config.yml")
+	if _, err := os.Stat(cfgPath); os.IsNotExist(err) {
+		cfg := &config.Config{
+			Engine: config.EngineConfig{
+				Version: currentVaultVersion,
+				VaultID: util.UUID7(),
+			},
+			Sync: config.SyncConfig{
+				AutoSync: false,
+			},
+			Browser: config.BrowserConfig{
+				Enabled:   false,
+				LocalPort: 47731,
+			},
+		}
+		if err := config.Save(vaultRoot, cfg); err != nil {
+			return fmt.Errorf("write config: %w", err)
+		}
+	}
+
+	dbPath := filepath.Join(vaultRoot, ".verstak", "index.db")
+	if _, err := os.Stat(dbPath); os.IsNotExist(err) {
+		db, err := storage.Open(dbPath)
+		if err != nil {
+			return fmt.Errorf("create db: %w", err)
+		}
+		db.Close()
+	}
+
+	return nil
+}