changelog-format

# CHANGELOG format Authority: the top of `CHANGELOG.md` (the newest block is the template), `scripts/dod_changelog.py` (DoD 3, the gate), CLAUDE.md "Output Rules" (the CHANGELOG voice is the one deliberate carve-out: entries ARGUE). - Heading: `## [X.Y.Z] - YYYY-MM-DD - <thesis sentence>`; the thesis is the one lesson of the release. `## [Unreleased]` sits above it and is never empty at release time. - An entry says what was wrong, why (the mechanism), and what the fix makes impossible, not what files changed. Credit the reporter by login and cite `#N`. - A number in an entry is pasted from an evidence file of the same run or read from `harness/thresholds.json`; a typed number is a finding for the reviewer. - A tool added or removed is named in the entry (`scripts/surface_diff.py` checks the CHANGELOG diff for the name). - A retired test's entry names the lesson and the replacement assertion (mirrors `harness/retired.json`, DoD 11). - The `no-changelog` label exempts a PR and is printed in the check summary; docs-only, CI-only and test-only changes trigger nothing.