Make cleanup-artifacts repository scope truly optional

[chubes4]

Problem

workspace worktree cleanup-artifacts describes its positional repository/handle scope as optional and documents no-repository examples, but WP-CLI renders <repo> as required because the structured positional synopsis is not marked optional. The handler and ability support unscoped bounded workspace review.

Acceptance

• Native help renders [<repo>].
• The documented no-repo dry-run dispatches successfully.
• Explicit repository and exact-handle scopes retain behavior.
• Command-generation and native WP-CLI synopsis tests cover the copy-paste contract.

AI assistance

GPT-5.6-sol via OpenCode reproduced the synopsis mismatch, audited command routing, and drafted this report. Chris Huber remains responsible.