feat(storage): add pinned field to issues schema

Add pinned column to the issues table to support persistent context markers
that should not be treated as work items (bd-7h5).

Changes:
- Add pinned column to schema.go CREATE TABLE
- Add migration 023_pinned_column.go for existing databases
- Update all issue queries to include pinned column
- Update scanIssues and scanIssuesWithDependencyType to handle pinned field
- Add Pinned field to types.Issue struct with JSON serialization
- Fix migrations_test.go to include pinned in legacy schema test

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

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

internal/storage/sqlite/labels.go

@@ -159,7 +159,7 @@ func (s *SQLiteStorage) GetIssuesByLabel(ctx context.Context, label string) ([]*
 		       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
 		JOIN labels l ON i.id = l.issue_id
 		WHERE l.label = ?