feat: add supermodel --update flag for self-update

[jonathanpopham]

Running `supermodel version` showed v0.1.0 while v0.4.2 was released. There's no way to update from the CLI itself.

Add `supermodel --update` (or `supermodel update`) that:

1. Checks the latest release on GitHub
2. Downloads the binary for the current platform
3. Replaces the installed binary

This is the same pattern as `gh upgrade`, `claude update`, etc.

Could reuse the install script: `curl -fsSL https://supermodeltools.com/install.sh | sh`