Fix remaining test database initialization errors (bd-207)

Fixed 38 tests failing with 'database not initialized: issue_prefix config is missing' by replacing manual sqlite.New() calls with test helper functions.

Modified files:
- dep_test.go (4 tests)
- merge_test.go (4 tests)
- export_import_test.go (4 instances)
- import_collision_test.go (10 instances)
- import_bug_test.go (1 instance)
- import_collision_regression_test.go (2 instances)
- import_idempotent_test.go (2 instances)
- init_test.go (4 instances)
- integrity_test.go (3 tests)
- main_test.go (multiple tests)

All database initialization errors are now resolved.
Remaining test failures (2) are unrelated to database initialization.

Amp-Thread-ID: https://ampcode.com/threads/T-a6b09458-b899-49eb-9a62-346fa67f62c7
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/import_bug_test.go

@@ -7,7 +7,6 @@ import (
 	"path/filepath"
 	"testing"
 
-	"github.com/steveyegge/beads/internal/storage/sqlite"
 	"github.com/steveyegge/beads/internal/types"
 )
 
@@ -22,16 +21,8 @@ func TestImportReturnsCorrectCounts(t *testing.T) {
 	defer os.RemoveAll(tmpDir)
 
 	dbPath := filepath.Join(tmpDir, ".beads", "issues.db")
-	if err := os.MkdirAll(filepath.Dir(dbPath), 0755); err != nil {
-		t.Fatalf("Failed to create .beads dir: %v", err)
-	}
-
 	// Initialize database
-	store, err := sqlite.New(dbPath)
-	if err != nil {
-		t.Fatalf("Failed to create store: %v", err)
-	}
-	defer store.Close()
+	store := newTestStore(t, dbPath)
 
 	ctx := context.Background()