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/storage/migrations.go

@@ -0,0 +1,29 @@
+package storage
+
+// migration001 — initial schema: nodes + node_meta.
+const migration001 = `
+CREATE TABLE IF NOT EXISTS nodes (
+    id TEXT PRIMARY KEY,
+    parent_id TEXT NULL REFERENCES nodes(id),
+    type TEXT NOT NULL,
+    title TEXT NOT NULL,
+    slug TEXT NOT NULL,
+    path TEXT NULL,
+    sort_order INTEGER NOT NULL DEFAULT 0,
+    created_at TEXT NOT NULL,
+    updated_at TEXT NOT NULL,
+    deleted_at TEXT NULL,
+    revision INTEGER NOT NULL DEFAULT 1,
+    device_id TEXT NULL
+);
+
+CREATE TABLE IF NOT EXISTS node_meta (
+    node_id TEXT NOT NULL REFERENCES nodes(id),
+    key TEXT NOT NULL,
+    value TEXT NOT NULL,
+    PRIMARY KEY (node_id, key)
+);
+
+CREATE INDEX IF NOT EXISTS idx_nodes_parent ON nodes(parent_id);
+CREATE INDEX IF NOT EXISTS idx_nodes_deleted ON nodes(deleted_at);
+`

internal/core/storage/storage.go

@@ -0,0 +1,120 @@
+package storage
+
+import (
+	"database/sql"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+// DB wraps sql.DB with Verstak-specific operations.
+type DB struct {
+	*sql.DB
+	path string
+}
+
+// Open opens or creates a SQLite database at path and runs pending migrations.
+func Open(path string) (*DB, error) {
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0o750); err != nil {
+		return nil, fmt.Errorf("create db dir: %w", err)
+	}
+
+	db, err := sql.Open("sqlite3", fmt.Sprintf("file:%s?mode=rwc", path))
+	if err != nil {
+		return nil, fmt.Errorf("open sqlite: %w", err)
+	}
+	db.SetMaxOpenConns(1)
+
+	w := &DB{DB: db, path: path}
+	if err := w.runInitialSchema(); err != nil {
+		db.Close()
+		return nil, err
+	}
+	if err := w.runMigrations(); err != nil {
+		db.Close()
+		return nil, err
+	}
+	return w, nil
+}
+
+// Path returns the on-disk path of the database file.
+func (db *DB) Path() string {
+	return db.path
+}
+
+// --- internals ---
+
+const schemaVersionDDL = `
+CREATE TABLE IF NOT EXISTS _schema_ver (
+    version INTEGER PRIMARY KEY,
+    applied_at TEXT NOT NULL
+);
+`
+
+var migrationFiles = map[int]string{
+	1: migration001,
+	// 2: migration002, etc.
+}
+
+func (db *DB) runInitialSchema() error {
+	_, err := db.Exec(schemaVersionDDL)
+	return err
+}
+
+func (db *DB) runMigrations() error {
+	var currentVer int
+	if err := db.QueryRow("SELECT COALESCE(MAX(version), 0) FROM _schema_ver").Scan(&currentVer); err != nil {
+		return err
+	}
+
+	// Build sorted version list.
+	versions := make([]int, 0, len(migrationFiles))
+	for v := range migrationFiles {
+		versions = append(versions, v)
+	}
+	for i := 0; i < len(versions); i++ {
+		for j := i + 1; j < len(versions); j++ {
+			if versions[j] < versions[i] {
+				versions[i], versions[j] = versions[j], versions[i]
+			}
+		}
+	}
+
+	for _, v := range versions {
+		if v <= currentVer {
+			continue
+		}
+		if err := db.applyMigration(v, migrationFiles[v]); err != nil {
+			return fmt.Errorf("migrate v%d: %w", v, err)
+		}
+	}
+	return nil
+}
+
+func (db *DB) applyMigration(version int, raw string) error {
+	tx, err := db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	for _, stmt := range strings.Split(raw, ";") {
+		stmt = strings.TrimSpace(stmt)
+		if stmt == "" {
+			continue
+		}
+		if _, err := tx.Exec(stmt); err != nil {
+			return fmt.Errorf("exec stat: %w; sql=%.200s", err, stmt)
+		}
+	}
+	if _, err := tx.Exec("INSERT INTO _schema_ver (version, applied_at) VALUES (?, ?)",
+		version, time.Now().UTC().Format(time.RFC3339)); err != nil {
+		return err
+	}
+	return tx.Commit()
+}