Phase 7.1 — GA cutover: composer rename, GitHub move, Packagist publish, v4.0.0 tag

[drewangell]

Plan file: docs/v4-plans/phase-7/01-ga-cutover.md

PRD: docs/PRD.md

This issue tracks execution of the plan file above. The plan file is the source of truth for scope, acceptance criteria, and verification — pick this issue up when ready to take the iteration, execute against the plan, and close when the acceptance criteria are met.

Phase 7 — Release & Brand Cutover

Links above are relative paths. They resolve on main after PR #285 merges (the PR adds both the revised PRD and the docs/v4-plans/ tree).