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/transaction.go

@@ -306,7 +306,7 @@ func (t *sqliteTxStorage) GetIssue(ctx context.Context, id string) (*types.Issue
 		       created_at, updated_at, closed_at, external_ref,
 		       compaction_level, compacted_at, compacted_at_commit, original_size, source_repo, close_reason,
 		       deleted_at, deleted_by, delete_reason, original_type,
-		       sender, ephemeral
+		       sender, ephemeral, pinned
 		FROM issues
 		WHERE id = ?
 	`, id)
@@ -1098,7 +1098,7 @@ func (t *sqliteTxStorage) SearchIssues(ctx context.Context, query string, filter
 		       created_at, updated_at, closed_at, external_ref,
 		       compaction_level, compacted_at, compacted_at_commit, original_size, source_repo, close_reason,
 		       deleted_at, deleted_by, delete_reason, original_type,
-		       sender, ephemeral
+		       sender, ephemeral, pinned
 		FROM issues
 		%s
 		ORDER BY priority ASC, created_at DESC
@@ -1141,6 +1141,8 @@ func scanIssueRow(row scanner) (*types.Issue, error) {
 	// Messaging fields (bd-kwro)
 	var sender sql.NullString
 	var ephemeral sql.NullInt64
+	// Protection fields (bd-b2k)
+	var pinned sql.NullInt64
 
 	err := row.Scan(
 		&issue.ID, &contentHash, &issue.Title, &issue.Description, &issue.Design,
@@ -1149,7 +1151,7 @@ func scanIssueRow(row scanner) (*types.Issue, error) {
 		&issue.CreatedAt, &issue.UpdatedAt, &closedAt, &externalRef,
 		&issue.CompactionLevel, &compactedAt, &compactedAtCommit, &originalSize, &sourceRepo, &closeReason,
 		&deletedAt, &deletedBy, &deleteReason, &originalType,
-		&sender, &ephemeral,
+		&sender, &ephemeral, &pinned,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("failed to scan issue: %w", err)
@@ -1203,6 +1205,10 @@ func scanIssueRow(row scanner) (*types.Issue, error) {
 	if ephemeral.Valid && ephemeral.Int64 != 0 {
 		issue.Ephemeral = true
 	}
+	// Protection fields (bd-b2k)
+	if pinned.Valid && pinned.Int64 != 0 {
+		issue.Pinned = true
+	}
 
 	return &issue, nil
 }