docs/publish-issue-format.md contract

docs/publish-issue-format.md is the formal Publish issue protocol, linked from README.md. Keep it synchronized with parser/controller behavior: title forms, optional legacy repository parsing, workspace JSON constraints, Requested by, optional Merge target, and ### Targets checkbox semantics. Chose a dedicated reference over README-only prose because workflow authors and controller maintainers need one stable, precise contract. Checked target entries persist across refreshes and reflect secure Craft state after retry; accepted starts publishing and dry-run selects dry-run mode.