feat: support installing skills individually

[christso]

Why

currently when installing plugins it installs all skills and other artifacts, and users need to manually remove the skills.
Often the user only needs a couple of skills or even one skill from a plugin.
Adding everything else in the plugin pollutes the agent's context.

Also, running allagents plugin install <pluginname> --skill skillname does not work when the plugin is already installed. I know we have allagens skills add but it doesn't seem to behave the same as npx skills add.

Proposal

Add feature to install the skill directly, regardless of whether the plugin is already installed or not.

Also review whether the schema where disabledSkills and enabledSkills is industry best practice, intuitive and composable in AI native paradigm.

use agent-tui to understand the command surface and UX for npx skills and ensure that npx allagents skills has feature parity while maintaining compatibility with allagents plugin and workspace approach.