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_idempotent_test.go

@@ -8,7 +8,6 @@ import (
 	"testing"
 	"time"
 
-	"github.com/steveyegge/beads/internal/storage/sqlite"
 	"github.com/steveyegge/beads/internal/types"
 )
 
@@ -230,11 +229,7 @@ func TestIdempotentImportNoTimestampChurn(t *testing.T) {
 	jsonlPath := filepath.Join(tmpDir, "issues.jsonl")
 
 	// Create store
-	testStore, err := sqlite.New(dbPath)
-	if err != nil {
-		t.Fatalf("Failed to create storage: %v", err)
-	}
-	defer testStore.Close()
+	testStore := newTestStoreWithPrefix(t, dbPath, "bd")
 
 	store = testStore
 	storeMutex.Lock()
@@ -316,11 +311,7 @@ func TestImportMultipleUnchangedIssues(t *testing.T) {
 	jsonlPath := filepath.Join(tmpDir, "issues.jsonl")
 
 	// Create store
-	testStore, err := sqlite.New(dbPath)
-	if err != nil {
-		t.Fatalf("Failed to create storage: %v", err)
-	}
-	defer testStore.Close()
+	testStore := newTestStoreWithPrefix(t, dbPath, "bd")
 
 	store = testStore
 	storeMutex.Lock()