feat(cleanup): protect pinned issues from cleanup/compact (bd-b2k)

Add Pinned field to Issue struct and database schema to protect
issues from accidental deletion via cleanup or compaction.

Changes:
- Add Pinned bool field to types.Issue
- Create migration 023_pinned_column.go for database schema
- Filter out pinned issues in cleanup command before deletion
- Add pinned check to GetTier1Candidates and GetTier2Candidates
- Add pinned check to CheckEligibility for compaction
- Update all SQL queries and scan functions to include pinned field

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/ready.go

@@ -101,7 +101,7 @@ func (s *SQLiteStorage) GetReadyWork(ctx context.Context, filter types.WorkFilte
 		i.status, i.priority, i.issue_type, i.assignee, i.estimated_minutes,
 		i.created_at, i.updated_at, i.closed_at, i.external_ref, i.source_repo, i.close_reason,
 		i.deleted_at, i.deleted_by, i.delete_reason, i.original_type,
-		i.sender, i.ephemeral
+		i.sender, i.ephemeral, i.pinned
 		FROM issues i
 		WHERE %s
 		AND NOT EXISTS (
@@ -130,7 +130,7 @@ func (s *SQLiteStorage) GetStaleIssues(ctx context.Context, filter types.StaleFi
 			created_at, updated_at, closed_at, external_ref, source_repo,
 			compaction_level, compacted_at, compacted_at_commit, original_size, close_reason,
 			deleted_at, deleted_by, delete_reason, original_type,
-			sender, ephemeral
+			sender, ephemeral, pinned
 		FROM issues
 		WHERE status != 'closed'
 		  AND datetime(updated_at) < datetime('now', '-' || ? || ' days')
@@ -179,6 +179,8 @@ func (s *SQLiteStorage) GetStaleIssues(ctx context.Context, filter types.StaleFi
 		// Messaging fields (bd-kwro)
 		var sender sql.NullString
 		var ephemeral sql.NullInt64
+		// Protection fields (bd-b2k)
+		var pinned sql.NullInt64
 
 		err := rows.Scan(
 			&issue.ID, &contentHash, &issue.Title, &issue.Description, &issue.Design,
@@ -187,7 +189,7 @@ func (s *SQLiteStorage) GetStaleIssues(ctx context.Context, filter types.StaleFi
 			&issue.CreatedAt, &issue.UpdatedAt, &closedAt, &externalRef, &sourceRepo,
 			&compactionLevel, &compactedAt, &compactedAtCommit, &originalSize, &closeReason,
 			&deletedAt, &deletedBy, &deleteReason, &originalType,
-			&sender, &ephemeral,
+			&sender, &ephemeral, &pinned,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("failed to scan stale issue: %w", err)
@@ -244,6 +246,10 @@ func (s *SQLiteStorage) GetStaleIssues(ctx context.Context, filter types.StaleFi
 		if ephemeral.Valid && ephemeral.Int64 != 0 {
 			issue.Ephemeral = true
 		}
+		// Protection fields (bd-b2k)
+		if pinned.Valid && pinned.Int64 != 0 {
+			issue.Pinned = true
+		}
 
 		issues = append(issues, &issue)
 	}