feat(tombstone): add P2 code review improvements (bd-saa, bd-1bu, bd-nyt)

- Add partial index on deleted_at for efficient TTL queries
- Exclude tombstones from SearchIssues by default (new IncludeTombstones filter)
- Report tombstone count separately in GetStatistics
- Display tombstone count in bd stats output

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

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

internal/types/types.go

@@ -297,6 +297,7 @@ type Statistics struct {
 	ClosedIssues             int     `json:"closed_issues"`
 	BlockedIssues            int     `json:"blocked_issues"`
 	ReadyIssues              int     `json:"ready_issues"`
+	TombstoneIssues          int     `json:"tombstone_issues"` // Soft-deleted issues (bd-nyt)
 	EpicsEligibleForClosure  int     `json:"epics_eligible_for_closure"`
 	AverageLeadTime          float64 `json:"average_lead_time_hours"`
 }
@@ -334,6 +335,9 @@ type IssueFilter struct {
 	// Numeric ranges
 	PriorityMin *int
 	PriorityMax *int
+
+	// Tombstone filtering (bd-1bu)
+	IncludeTombstones bool // If false (default), exclude tombstones from results
 }
 
 // SortPolicy determines how ready work is ordered