test: add comprehensive orphan handling mode tests

- TestOrphanHandling_Strict: Verifies import fails on missing parent
- TestOrphanHandling_Resurrect: Verifies parent tombstone creation
- TestOrphanHandling_Skip: Verifies orphans are skipped with warning
- TestOrphanHandling_Allow: Verifies orphans import without validation
- TestOrphanHandling_Config: Tests config reading with all modes + defaults
- TestOrphanHandling_NonHierarchical: Verifies flat IDs work in all modes

Also fixes batch_ops_test.go to pass OrphanHandling parameter to generateBatchIDs.

All tests pass. Closes bd-968f

Amp-Thread-ID: https://ampcode.com/threads/T-fd18d4a5-06b3-4400-9073-194d570846d8
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/batch_ops_test.go

@@ -270,7 +270,7 @@ func TestGenerateBatchIDs(t *testing.T) {
 			{Title: "Issue 3", Description: "Third", CreatedAt: time.Now()},
 		}
 
-		err = s.generateBatchIDs(ctx, conn, issues, "test-actor")
+		err = s.generateBatchIDs(ctx, conn, issues, "test-actor", OrphanAllow)
 		if err != nil {
 			t.Fatalf("failed to generate IDs: %v", err)
 		}
@@ -299,7 +299,7 @@ func TestGenerateBatchIDs(t *testing.T) {
 			{ID: "wrong-prefix-123", Title: "Wrong", CreatedAt: time.Now()},
 		}
 
-		err = s.generateBatchIDs(ctx, conn, issues, "test-actor")
+		err = s.generateBatchIDs(ctx, conn, issues, "test-actor", OrphanAllow)
 		if err == nil {
 			t.Fatal("expected error for wrong prefix")
 		}