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/config/config.go

@@ -0,0 +1,68 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Config lives at .verstak/config.yml inside the vault.
+type Config struct {
+	Engine      EngineConfig `yaml:"engine"`
+	Sync        SyncConfig   `yaml:"sync"`
+	Browser     BrowserConfig `yaml:"browser"`
+}
+
+type EngineConfig struct {
+	Version     int    `yaml:"version"`
+	VaultID     string `yaml:"vault_id"`
+	CreatedAt   string `yaml:"created_at"`
+	VaultRoot   string `yaml:"vault_root"`
+}
+
+type SyncConfig struct {
+	ServerURL   string `yaml:"server_url"`
+	APIKey      string `yaml:"api_key"`
+	DeviceID    string `yaml:"device_id"`
+	AutoSync    bool   `yaml:"auto_sync"`
+}
+
+type BrowserConfig struct {
+	Enabled     bool   `yaml:"enabled"`
+	LocalPort   int    `yaml:"local_port"`
+}
+
+// Load reads .verstak/config.yml from the vault root.
+func Load(vaultRoot string) (*Config, error) {
+	path := filepath.Join(vaultRoot, ".verstak", "config.yml")
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read config: %w", err)
+	}
+	var cfg Config
+	if err := yaml.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parse config: %w", err)
+	}
+	return &cfg, nil
+}
+
+// Save writes the config file.
+func Save(vaultRoot string, cfg *Config) error {
+	path := filepath.Join(vaultRoot, ".verstak", "config.yml")
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0o750); err != nil {
+		return err
+	}
+	data, err := yaml.Marshal(cfg)
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(path, data, 0o640)
+}
+
+// MetaPath returns the path inside the vault for sqlite db etc.
+func MetaDir(vaultRoot string) string {
+	return filepath.Join(vaultRoot, ".verstak")
+}