THE-14: add release pipeline and rollback plan

[ConduciveMammal]

Summary

• add a formal release pipeline runbook for candidate preparation, validation, CI artefacts, and sign-off
• add a rollback plan with triggers, execution checklist, and recovery exit criteria
• update the QA Release Candidate workflow to create and upload a versioned build artefact zip
• update the release checklist to reference the new runbooks and THE-14 evidence

Files changed

• .github/workflows/qa-release-candidate.yml
• docs/qa/release-checklist.md
• docs/qa/release-pipeline.md
• docs/qa/rollback-plan.md

Validation

• npm run qa:release-candidate