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/storage/sqlite/migrations/018_tombstone_columns.go

@@ -16,7 +16,7 @@ func MigrateTombstoneColumns(db *sql.DB) error {
 		name         string
 		definition   string
 	}{
-		{"deleted_at", "TEXT"},
+		{"deleted_at", "DATETIME"},
 		{"deleted_by", "TEXT DEFAULT ''"},
 		{"delete_reason", "TEXT DEFAULT ''"},
 		{"original_type", "TEXT DEFAULT ''"},