feat: add registry command to manage global and local skill registries, drop built-in registries

[jakemac53]

This includes various registry related improvements and a new set of commands:

• Adds registry add, registry list, and registry remove commands.
  • These support both git URIs and owner/repo formats (which infers GitHub), as trailing arguments. Multiple registries can be added/removed at time.
• Adds a global config file to store permanent configuration for a machine.
• Adds support for globally installed registries as well as project level registries.
• Adds support for running global migrations, and adds a migration for existing projects. It will ask if they want to retain the existing registries or not. This supports a graceful migration for existing users.
• The API can still just provide its own list of registries, so serverpod can automatically provide registries if it wants. But the core package does not have any built in registries.
• Registry layout is now inferred and not hard coded in the config.

Closes dart-lang/ai#463

Note that this PR and #14 are stepping on each toes a bit, whichever lands first though I can reconcile the other one.