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_shared.go

@@ -165,6 +165,7 @@ type ImportOptions struct {
 	SkipPrefixValidation       bool   // Skip prefix validation (for auto-import)
 	ClearDuplicateExternalRefs bool   // Clear duplicate external_ref values instead of erroring
 	OrphanHandling             string // Orphan handling mode: strict/resurrect/skip/allow (empty = use config)
+	NoGitHistory               bool   // Skip git history backfill for deletions (prevents spurious deletion during JSONL migrations)
 }
 
 // ImportResult contains statistics about the import operation
@@ -221,6 +222,7 @@ func importIssuesCore(ctx context.Context, dbPath string, store storage.Storage,
 		SkipPrefixValidation:       opts.SkipPrefixValidation,
 		ClearDuplicateExternalRefs: opts.ClearDuplicateExternalRefs,
 		OrphanHandling:             importer.OrphanHandling(orphanHandling),
+		NoGitHistory:               opts.NoGitHistory,
 	}
 
 	// Delegate to the importer package