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/types/types.go

@@ -34,6 +34,11 @@ type Issue struct {
 	Labels             []string       `json:"labels,omitempty"` // Populated only for export/import
 	Dependencies       []*Dependency  `json:"dependencies,omitempty"` // Populated only for export/import
 	Comments           []*Comment     `json:"comments,omitempty"`     // Populated only for export/import
+	// Tombstone fields (bd-vw8): inline soft-delete support
+	DeletedAt     *time.Time `json:"deleted_at,omitempty"`     // When the issue was deleted
+	DeletedBy     string     `json:"deleted_by,omitempty"`     // Who deleted the issue
+	DeleteReason  string     `json:"delete_reason,omitempty"`  // Why the issue was deleted
+	OriginalType  string     `json:"original_type,omitempty"`  // Issue type before deletion (for tombstones)
 }
 
 // ComputeContentHash creates a deterministic hash of the issue's content.
@@ -69,6 +74,11 @@ func (i *Issue) ComputeContentHash() string {
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
 
+// IsTombstone returns true if the issue has been soft-deleted (bd-vw8)
+func (i *Issue) IsTombstone() bool {
+	return i.Status == StatusTombstone
+}
+
 // Validate checks if the issue has valid field values (built-in statuses only)
 func (i *Issue) Validate() error {
 	return i.ValidateWithCustomStatuses(nil)
@@ -114,12 +124,13 @@ const (
 	StatusInProgress Status = "in_progress"
 	StatusBlocked    Status = "blocked"
 	StatusClosed     Status = "closed"
+	StatusTombstone  Status = "tombstone" // Soft-deleted issue (bd-vw8)
 )
 
 // IsValid checks if the status value is valid (built-in statuses only)
 func (s Status) IsValid() bool {
 	switch s {
-	case StatusOpen, StatusInProgress, StatusBlocked, StatusClosed:
+	case StatusOpen, StatusInProgress, StatusBlocked, StatusClosed, StatusTombstone:
 		return true
 	}
 	return false