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.

migrations/001_init.sql

@@ -0,0 +1,24 @@
+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);