feat(db): add SQLite schema, Store CRUD, ULID generation

Foundation layer: entities table with card support, entity_tags join
table, WAL mode, busy_timeout, full CRUD operations including
promote/demote lifecycle and soft/hard delete. 33 tests passing.

internal/ulid/ulid.go

@@ -0,0 +1,20 @@
+package ulid
+
+import (
+	"crypto/rand"
+	"sync"
+
+	"github.com/oklog/ulid/v2"
+)
+
+var (
+	entropy     *ulid.MonotonicEntropy
+	entropyOnce sync.Once
+)
+
+func New() string {
+	entropyOnce.Do(func() {
+		entropy = ulid.Monotonic(rand.Reader, 0)
+	})
+	return ulid.MustNew(ulid.Now(), entropy).String()
+}