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>

cmd/bd/import.go

@@ -84,6 +84,7 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 		clearDuplicateExternalRefs, _ := cmd.Flags().GetBool("clear-duplicate-external-refs")
 		orphanHandling, _ := cmd.Flags().GetString("orphan-handling")
 		force, _ := cmd.Flags().GetBool("force")
+		noGitHistory, _ := cmd.Flags().GetBool("no-git-history")
 
 		// Check if stdin is being used interactively (not piped)
 		if input == "" && term.IsTerminal(int(os.Stdin.Fd())) {
@@ -242,6 +243,7 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 			RenameOnImport:             renameOnImport,
 			ClearDuplicateExternalRefs: clearDuplicateExternalRefs,
 			OrphanHandling:             orphanHandling,
+			NoGitHistory:               noGitHistory,
 		}
 
 		result, err := importIssuesCore(ctx, dbPath, store, allIssues, opts)
@@ -743,6 +745,7 @@ func init() {
 	importCmd.Flags().Bool("clear-duplicate-external-refs", false, "Clear duplicate external_ref values (keeps first occurrence)")
 	importCmd.Flags().String("orphan-handling", "", "How to handle missing parent issues: strict/resurrect/skip/allow (default: use config or 'allow')")
 	importCmd.Flags().Bool("force", false, "Force metadata update even when database is already in sync with JSONL")
+	importCmd.Flags().Bool("no-git-history", false, "Skip git history backfill for deletions (use during JSONL filename migrations)")
 	importCmd.Flags().BoolVar(&jsonOutput, "json", false, "Output import statistics in JSON format")
 	rootCmd.AddCommand(importCmd)
 }