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>

.changeset/935-changeset-cli-install.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 935
+---
+**`/gsd-update` changelog preview no longer silently fails** — the installer now copies `scripts/changeset/` and `scripts/lib/` into the runtime config dir so `$GSD_DIR/scripts/changeset/cli.cjs` resolves at runtime; `update.md` was updated to use the correct installed path and to surface an explicit error if the CLI is missing rather than swallowing it.