feat(import,sync): add --no-git-history flag to prevent spurious deletions

Fixes bd-0b2: The git history backfill mechanism was causing data loss
during JSONL filename migrations (beads.jsonl → issues.jsonl). When issues
existed in the old filename's git history, the backfill incorrectly treated
them as "deleted" and purged them from the database.

Changes:
- Add NoGitHistory field to importer.Options and ImportOptions structs
- Modify purgeDeletedIssues() to skip git history check when flag is set
- Add --no-git-history flag to bd import command
- Add --no-git-history flag to bd sync command
- Update purge_test.go to pass Options argument

Usage:
  bd import -i .beads/issues.jsonl --no-git-history
  bd sync --no-git-history

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/importer/purge_test.go

@@ -80,7 +80,7 @@ func TestPurgeDeletedIssues(t *testing.T) {
 	}
 
 	// Call purgeDeletedIssues
-	if err := purgeDeletedIssues(ctx, store, dbPath, jsonlIssues, result); err != nil {
+	if err := purgeDeletedIssues(ctx, store, dbPath, jsonlIssues, Options{}, result); err != nil {
 		t.Fatalf("purgeDeletedIssues failed: %v", err)
 	}
 
@@ -159,7 +159,7 @@ func TestPurgeDeletedIssues_NoDeletionsManifest(t *testing.T) {
 	}
 
 	// Call purgeDeletedIssues - should succeed with no errors
-	if err := purgeDeletedIssues(ctx, store, dbPath, jsonlIssues, result); err != nil {
+	if err := purgeDeletedIssues(ctx, store, dbPath, jsonlIssues, Options{}, result); err != nil {
 		t.Fatalf("purgeDeletedIssues failed: %v", err)
 	}
 
@@ -222,7 +222,7 @@ func TestPurgeDeletedIssues_EmptyDeletionsManifest(t *testing.T) {
 	}
 
 	// Call purgeDeletedIssues - should succeed with no errors
-	if err := purgeDeletedIssues(ctx, store, dbPath, jsonlIssues, result); err != nil {
+	if err := purgeDeletedIssues(ctx, store, dbPath, jsonlIssues, Options{}, result); err != nil {
 		t.Fatalf("purgeDeletedIssues failed: %v", err)
 	}