docs(changelog): record the version-guard CHANGELOG-heading requirement

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## [Unreleased]
 
+### Added
+- **`version-guard` now requires every version bump to be documented under a real `## [X.Y.Z]` CHANGELOG heading.** The guard already refused a bump to a version that was already on npm, but it never checked that the new version was described anywhere. Notes parked under `## [Unreleased]` are orphaned the moment the release ships: nothing in the release path renames that section — the `version` lifecycle script only syncs the plugin manifests — so the published version goes out undocumented while its release notes sit under a heading still claiming they are unreleased. **This repo is where that actually happened**: 2.6.10 and 2.6.11 both shipped with their notes stranded under `## [Unreleased]`, and were only filed under real headings retroactively. A bump whose version has no matching heading now hard-fails the PR, with an error naming the heading to add. Keep an empty `## [Unreleased]` at the top regardless — `dependabot-rebuild.yml` hard-exits without that marker, and since it already inserts a real heading, bot PRs pass unchanged. The guard file lives in `.github/`, which does not ship, so this owes no version bump. Matches apple-mail-mcp#124, keeping the guard identical across the four servers. (#117)
+
 ## [2.6.12] - 2026-08-01
 
 ### Fixed