When GSD is installed with `bun add -g`, running `gsd update` or
`/gsd update` previously shelled out to `npm install -g`, which fails
with EACCES on systems where npm has no write access to the global
node_modules directory.
Adds `resolveInstallCommand(pkg)` to `update-check.ts` that returns
`bun add -g <pkg>` when `process.versions.bun` is defined (i.e. the
current runtime is Bun), and `npm install -g <pkg>` otherwise. All
three update paths — `update-cmd.ts`, `commands-handlers.ts`, and the
interactive startup prompt in `update-check.ts` — now use this helper,
including the fallback error message shown to the user.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds a CLI subcommand that checks npm for the latest version and
runs `npm install -g gsd-pi@latest` if an update is available.
Prints current/latest version and clear success/failure messages.
