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/db/db_test.go

@@ -0,0 +1,80 @@
+package db
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func testStore(t *testing.T) *Store {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "test.db")
+	s, err := Open(path)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	t.Cleanup(func() { s.Close() })
+	return s
+}
+
+func TestOpen_CreatesFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "test.db")
+	s, err := Open(path)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	defer s.Close()
+
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		t.Fatal("database file not created")
+	}
+}
+
+func TestOpen_WALMode(t *testing.T) {
+	s := testStore(t)
+	var mode string
+	if err := s.db.QueryRow("PRAGMA journal_mode").Scan(&mode); err != nil {
+		t.Fatal(err)
+	}
+	if mode != "wal" {
+		t.Errorf("expected wal, got %s", mode)
+	}
+}
+
+func TestOpen_ForeignKeys(t *testing.T) {
+	s := testStore(t)
+	var fk int
+	if err := s.db.QueryRow("PRAGMA foreign_keys").Scan(&fk); err != nil {
+		t.Fatal(err)
+	}
+	if fk != 1 {
+		t.Errorf("expected foreign_keys=1, got %d", fk)
+	}
+}
+
+func TestOpen_MigrateIdempotent(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "test.db")
+	s1, err := Open(path)
+	if err != nil {
+		t.Fatalf("first Open: %v", err)
+	}
+	s1.Close()
+
+	s2, err := Open(path)
+	if err != nil {
+		t.Fatalf("second Open: %v", err)
+	}
+	s2.Close()
+}
+
+func TestDefaultPath_EnvOverride(t *testing.T) {
+	want := "/tmp/custom-nib.db"
+	t.Setenv("NIB_DB", want)
+	got, err := DefaultPath()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != want {
+		t.Errorf("expected %s, got %s", want, got)
+	}
+}