Expand comma lists and add confidential to created issues

What does this MR do and why?

This MR builds on Stop GraphQL rules losing labels, assignees, an... (!390 - merged) and merges after it.

• A label or assignee entry can now interpolate to a comma-separated list. This covers issues created by summarize: and issue:, and labels added or removed by an update. Each value is checked and sent on its own, the way update assignees already work. A policy can then build a list whose length depends on the resource. GitLab doesn't allow commas in label titles or usernames, so no real value gets split.
• A new confidential: key creates the issue confidential in the same call that creates it. It takes true, or a template that evaluates to "true". Any other value creates a public issue. Dry runs print * Confidential: true.
• The pre-write check also evaluates confidential:, so a template that raises fails before any action writes.