Support updating skills to latest/specific versions

[dutifulbob]

We should support updating installed skills to their latest version or a specific version. This could include:

• skill-install update <id> or skill-install --update for an installed skill
• ability to specify a version (e.g., --version 0.1.4) when installing/updating
• a way to check installed skill versions vs latest

If this is out of scope for the core CLI, we can define the spec for update/version handling and let implementations decide.