feat: issue list — add --assignee, --author, --milestone, --search flags

## Missing flags (4 high priority)

| Flag | Type | Gitea API param |
|------|------|----------------|
| --assignee | string | ?assigned_by= |
| --author | string | ?created_by= |
| --milestone | string | ?milestone= |
| --search | string | ?q= (extended) |

All supported by Gitea API.

Source: flag parity analysis