chore(docs): centralize 1.6.0 changelog updates in one place, not per-feature-PR #2449

Standing practice going forward (owner, 2026-10-30): feature PRs should NOT each add their own bullet to docs/source/changelog/1.6.0.rst. Every wave-2 PR touching that same file in parallel is exactly what produced repeated merge conflicts this round (#2441/#2443/#2444/#2446 all edited it; whichever merged last needed a hand-merge every time).

New practice: the changelog entry for each landed feature gets added centrally — by the maintainer/main-thread, in one place — once a feature PR is ready to merge or has merged, rather than by the feature PR's own branch. Feature-PR workers should be told explicitly not to touch docs/source/changelog/1.6.0.rst (or whichever version file is current) as part of their own diff.

This issue tracks that practice and serves as the home for any any cross-cutting changelog consolidation work. No code change — process only.