fix(#690): backfill CHANGELOG 1.3.x on main for /gsd:update preview (#696)

Fast-track of the CHANGELOG.md backfill from #694 directly to `main` so the
live `/gsd:update` "What's New" preview (which fetches main's CHANGELOG.md)
immediately shows the 1.3.0/1.3.1 release notes instead of the empty message.

CHANGELOG-only; identical content to the dated [1.3.0]/[1.3.1] sections merged
to `next` in #694. The code-side fixes (verify gate, update.md, tests) reach
main on the next release. Uses the fix/critical-* lane because it is the
sanctioned main-target path and auto-back-merges to next.

Closes #690

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,18 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+## [1.3.1](https://www.npmjs.com/package/@opengsd/gsd-core/v/1.3.1) - 2026-06-04
+
+### Security
+
+- **Bumped `hono` to clear a moderate npm advisory** carried transitively in the dependency tree. (#670)
+
+### Fixed
+
+- **Installer-migration checksum drift no longer blocks upgrades** — the updater now self-heals when a shipped migration's recorded checksum has drifted, reconciling the stored checksum instead of aborting. Restores upgrades across all OSes after shipped migration bodies were edited in a prior release. (#670)
+
+## [1.3.0](https://www.npmjs.com/package/@opengsd/gsd-core/v/1.3.0) - 2026-06-04
+
 ### Added
 
 - **Vertical MVP Slice mode** — `--mvp` flag on `/gsd-plan-phase` switches the planner from horizontal layer decomposition to vertical feature-slice decomposition (UI→API→DB in one task sequence). On Phase 1 of a new project with no prior phase summaries, also emits `SKELETON.md` via Walking Skeleton mode. Composable with `--tdd`: `--mvp --tdd` produces vertical slices where every behavior-adding task starts with a failing test. Phase-level persistence via `**Mode:** mvp` in ROADMAP.md applies `--mvp` automatically without the flag. (#78)