* feat(release): changelog.d/ fragments — kill the CHANGELOG-eat merge-storm cascade
Every PR used to edit the same top lines of CHANGELOG.md (its bullet), so in a
merge-storm each merge conflicted every sibling (CHANGELOG-eat / DIRTY cascade),
forcing a re-sync push + full CI re-run per PR per merge — O(N^2) CI runs.
A PR now adds ONE new file under changelog.d/{features|fixes|maintenance}/ with its
bullet; two PRs never touch the same file. scripts/release/aggregate-changelog.mjs
(npm run changelog:aggregate) folds fragments into the living section and deletes
them at release reconciliation. check:changelog-integrity (already wired in the
merge-integrity CI job — zero workflow change) now also validates fragment
well-formedness. This PR dogfoods the convention: its own entry is a fragment.
* chore(changelog): fragment filename matches PR number (#6783)
