fix(#690): promote CHANGELOG 1.3.x + gate release-notes promotion (#694)

* fix(#690): promote CHANGELOG 1.3.x + gate release-notes promotion

/gsd:update showed an empty "What's New" preview after updating to 1.3.1
because CHANGELOG.md's 1.3.x content was never promoted out of [Unreleased]
into dated sections, so `scripts/changeset/cli.cjs extract` returned exit 2
("no releases in range").

- CHANGELOG.md: split [Unreleased] into dated [1.3.0] and [1.3.1] sections
  (1.3.1 = hono advisory bump + installer-migration checksum self-heal, #670;
  1.3.0 = the feature release), restoring an empty [Unreleased].
- scripts/changeset/cli.cjs: new `verify` subcommand that exits non-zero when
  CHANGELOG has no dated `## [x.y.z]` heading for a version; hoist shared
  stripV/resolveChangelogPath helpers used by extract + verify.
- .github/workflows/release.yml: gate the finalize job on `verify` (after the
  build, before tag/publish) so an unpromoted CHANGELOG can never ship again.
- gsd-core/workflows/update.md: move `rm -f $CHANGELOG_TMP` after the
  human-readable extract re-run so the preview no longer degrades to
  "(changelog unavailable)".
- tests: regression guard for the 1.3.x headings + extract range + verify
  command coverage (present/absent/undated/v-prefixed/--json/prerelease).

Closes #690

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#690): add changeset fragment for #694

Fixed-type fragment for the user-facing /gsd:update preview fix and the
release-notes promotion gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

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

.changeset/tidy-otters-glide.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 694
+---
+**`/gsd:update` reliably previews release notes again** — promotes the 1.3.x changelog into dated `[1.3.0]`/`[1.3.1]` sections, stops deleting the temp changelog before the human-readable render (no more `(changelog unavailable)`), and adds a release gate that blocks publishing a version whose `CHANGELOG.md` section was never promoted.