feat(types): add tombstone support for inline soft-delete (bd-fbj)

Add tombstone types and schema migration as foundation for the tombstone
epic (bd-vw8) which replaces deletions.jsonl with inline tombstones.

Changes:
- Add tombstone fields to Issue struct: DeletedAt, DeletedBy, DeleteReason, OriginalType
- Add StatusTombstone constant and IsTombstone() helper method
- Update Status.IsValid() to accept tombstone status
- Create migration 018_tombstone_columns.go for new database columns
- Update schema.go with tombstone columns: deleted_at, deleted_by, delete_reason, original_type
- Update all issue insert/update/scan operations across:
  - issues.go (insertIssue, insertIssues)
  - queries.go (GetIssue, GetIssueByExternalRef, SearchIssues)
  - dependencies.go (scanIssues, scanIssuesWithDependencyType)
  - transaction.go (scanIssueRow, GetIssue, SearchIssues)
  - multirepo.go (import operations)
  - ready.go (GetReadyWork, GetStaleIssues)
  - labels.go (GetIssuesByLabel)
- Add test for IsTombstone() helper
- Update migration test to include tombstone columns

Unblocks: bd-olt (TTL logic), bd-3b4 (delete command), bd-0ih (merge updates), bd-dve (import/export)

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

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

internal/storage/sqlite/migrations.go

@@ -34,6 +34,7 @@ var migrationsList = []Migration{
 	{"blocked_issues_cache", migrations.MigrateBlockedIssuesCache},
 	{"orphan_detection", migrations.MigrateOrphanDetection},
 	{"close_reason_column", migrations.MigrateCloseReasonColumn},
+	{"tombstone_columns", migrations.MigrateTombstoneColumns},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -75,6 +76,7 @@ func getMigrationDescription(name string) string {
 		"blocked_issues_cache":         "Adds blocked_issues_cache table for GetReadyWork performance optimization (bd-5qim)",
 		"orphan_detection":             "Detects orphaned child issues and logs them for user action (bd-3852)",
 		"close_reason_column":          "Adds close_reason column to issues table for storing closure explanations (bd-uyu)",
+		"tombstone_columns":            "Adds tombstone columns (deleted_at, deleted_by, delete_reason, original_type) for inline soft-delete (bd-vw8)",
 	}
 	
 	if desc, ok := descriptions[name]; ok {