feat(tombstone): implement delete-to-tombstone and TTL expiration (bd-3b4, bd-olt)

Phase 1 of tombstone migration: bd delete now creates tombstones instead
of hard-deleting issues.

Key changes:
- Add CreateTombstone() method to SQLiteStorage for soft-delete
- Modify executeDelete() to create tombstones instead of removing rows
- Add IsExpired() method with 30-day default TTL and clock skew grace
- Fix deleted_at schema from TEXT to DATETIME for proper time scanning
- Update delete.go to call CreateTombstone (single issue path)
- Still writes to deletions.jsonl for backward compatibility (dual-write)
- Dependencies are removed when creating tombstones
- Tombstones are excluded from normal searches (bd-1bu)

TTL constants:
- DefaultTombstoneTTL: 30 days
- MinTombstoneTTL: 7 days (safety floor)
- ClockSkewGrace: 1 hour

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

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

internal/types/types.go

@@ -74,11 +74,47 @@ func (i *Issue) ComputeContentHash() string {
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
 
+// DefaultTombstoneTTL is the default time-to-live for tombstones (30 days)
+const DefaultTombstoneTTL = 30 * 24 * time.Hour
+
+// MinTombstoneTTL is the minimum allowed TTL (7 days) to prevent data loss
+const MinTombstoneTTL = 7 * 24 * time.Hour
+
+// ClockSkewGrace is added to TTL to handle clock drift between machines
+const ClockSkewGrace = 1 * time.Hour
+
 // IsTombstone returns true if the issue has been soft-deleted (bd-vw8)
 func (i *Issue) IsTombstone() bool {
 	return i.Status == StatusTombstone
 }
 
+// IsExpired returns true if the tombstone has exceeded its TTL.
+// Non-tombstone issues always return false.
+// ttl is the configured TTL duration; if zero, DefaultTombstoneTTL is used.
+func (i *Issue) IsExpired(ttl time.Duration) bool {
+	// Non-tombstones never expire
+	if !i.IsTombstone() {
+		return false
+	}
+
+	// Tombstones without DeletedAt are not expired (safety: shouldn't happen in valid data)
+	if i.DeletedAt == nil {
+		return false
+	}
+
+	// Use default TTL if not specified
+	if ttl == 0 {
+		ttl = DefaultTombstoneTTL
+	}
+
+	// Add clock skew grace period to the TTL
+	effectiveTTL := ttl + ClockSkewGrace
+
+	// Check if the tombstone has exceeded its TTL
+	expirationTime := i.DeletedAt.Add(effectiveTTL)
+	return time.Now().After(expirationTime)
+}
+
 // Validate checks if the issue has valid field values (built-in statuses only)
 func (i *Issue) Validate() error {
 	return i.ValidateWithCustomStatuses(nil)