[Phase 1] Release-cut + rollback runbook

[don-petry]

Parent: #495 · Phase 1 · Work type: Docs · Targets SC4.

Write the runbook for the channel-tag model: how to cut a release (vX.Y.Z), how to promote (move stable/ring tag forward), and how to roll back (move the tag back) in < 5 minutes with no per-caller edits and no manual file surgery.

Done when: runbook lives in docs/, validated by a dry-run promote + rollback (tag move + verify a caller picks up the change).

Ref: §5.1, §7.

[github-actions]

Thanks for opening this and linking it to the parent initiative! To help validate the runbook effectively, could you clarify what the expected entry point looks like — for example, is the dry-run rollback intended to run locally against a test repo, or will it be exercised as part of a CI workflow?