feat(import/export): add tombstone support (bd-dve)

Update import/export to handle tombstones for deletion sync propagation:

Exporter:
- Include tombstones in JSONL output by setting IncludeTombstones: true
- Both single-repo and multi-repo exports now include tombstones

Importer:
- Tombstones from JSONL are imported as-is (they're issues with status=tombstone)
- Legacy deletions.jsonl entries are converted to tombstones via convertDeletionToTombstone()
- Non-tombstone issues in deletions manifest are still skipped (backward compat)
- purgeDeletedIssues() now creates tombstones instead of hard-deleting

This is Phase 2 of the tombstone implementation (bd-dli design), enabling
inline soft-delete tracking for cross-clone deletion synchronization.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/export.go

@@ -179,7 +179,8 @@ Examples:
 		labelsAny = util.NormalizeLabels(labelsAny)
 
 		// Build filter
-		filter := types.IssueFilter{}
+		// Include tombstones in export for sync propagation (bd-dve)
+		filter := types.IssueFilter{IncludeTombstones: true}
 		if statusFilter != "" {
 			status := types.Status(statusFilter)
 			filter.Status = &status