fix(#935): install changeset CLI so /gsd-update changelog preview works (#938)

- bin/install.js now copies scripts/changeset/ and scripts/lib/ into
  <configDir>/scripts/ so $GSD_DIR/scripts/changeset/cli.cjs resolves
  at runtime; aborts install with an explicit failure if the source
  directory is missing from the package.
- gsd-core/workflows/update.md: corrected path from
  gsd-core/scripts/changeset/cli.cjs to scripts/changeset/cli.cjs;
  added an explicit [ ! -f ] guard so a missing CLI surfaces a clear
  message rather than silently swallowing the error; stderr captured
  via 2>&1 sentinel so node errors are visible in the preview output.
- release.yml's changeset-CLI invocations (node scripts/changeset/cli.cjs)
  remain at the repo-root path and are unaffected by this change.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/changeset/cli.cjs

@@ -324,7 +324,7 @@ function resolveChangelogPath(opts) {
  *   1  — I/O error or missing required flags.
  *
  * Fix for #3496: provides a deterministic range-aware helper so the
- * `/gsd:update` show_changes_and_confirm step no longer relies on
+ * `/gsd-update` show_changes_and_confirm step no longer relies on
  * vague/manual extraction that can silently skip intermediate versions.
  */
 function cmdExtract(opts) {