Add RELEASING.md + pre-push hook to enforce tag immutability.
No functional changes from 1.13.15 — this is a docs/process release that
documents the tag-immutability rule and adds an opt-in pre-push hook to
prevent accidental force-pushes to tags. See RELEASING.md.
Why a new tag instead of moving 1.13.15: moving tags breaks downstream
SPM Package.resolved pins. See Dasher-Apple PR #10 for the latest
incident.