docs: add sk installation instructions

[alizain]

sk is a universal package manager (works across all coding agents), handles updates, etc.. (npm/cargo for agents).

would you be open to adding this?

[snarktank]

@alizain interesting idea. Walk me through the user experience for someone, after you implemented it.

[alizain]

Probably the best way to get a feel for it.

Some notes:

• agents.toml is equivalent to package.json.
• you can define all kinds of dependencies. sk will automatically extract the skills out of them based on the dependency format
  • github repos with optional path, tag, branch, or ref parameters
  • git urls with optional path, tag, branch, or ref parameters
  • claude plugins
  • local paths
• in the case of this repo, it matched as a gh pkg type with a path of ./skills. therefore the installation command is sk pkg add snarktank/ralph --path skills. users can also run sk pkg add snarktank/ralph and sk will apply a best-effort to detect the dependency/package type prompt to select installable skills or collection .
• by default, sk works in project-scope. the --global flag makes it run in user scope (~/)
• i want to add support for commands, subagents, and other ways to extend coding agents. a common package spec seems useful. a skills marketplace is very exciting