Check for GSD updates, install if available, and display what changed.

Routes to the update workflow which handles:

• Version detection (local vs global installation)
• npm version checking
• Changelog fetching and display
• User confirmation with clean install warning
• Update execution and cache clearing
• Restart reminder

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- **--sync**: Sync managed GSD skills across runtime roots so multi-runtime users stay aligned after an update. Runs the sync-skills workflow (--from, --to, --dry-run, --apply flags supported). - **--reapply**: Reapply local modifications after a GSD update. Uses three-way comparison (pristine baseline, user-modified backup, newly installed version) to merge user customizations back. Runs the reapply-patches workflow. - **--next** (alias **--rc**): Target the `@next` RC dist-tag instead of `@latest` so you can install or refresh a release candidate (e.g. `1.4.0-rc.1`) through the normal update flow — scope/runtime detection, changelog preview, custom-file backup, and cache clearing all still apply. Omitting it keeps targeting `@latest` (no change). See ADR #660 for the RC channel. - **(no flag)**: Standard update — check for new version, show changelog, install. Parse the first token of $ARGUMENTS: - If it is `--sync`: strip the flag, execute the sync-skills workflow (passing remaining args for --from/--to/--dry-run/--apply). - If it is `--reapply`: strip the flag, execute the reapply-patches workflow. - Otherwise (including `--next` / `--rc`): execute the update workflow end-to-end, passing `$ARGUMENTS` through so the workflow's parse_update_channel step can select the release channel.

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