Add agent, command, and skill hooks for direct plugin registration

[ryanrhughes]

While trying to create an inclusive plugin that could contain a related set of items, I kept having to take hackish routes to get skills and commands populated.

This PR seeks to remedy that by adding hooks such that a plugin could be capable of shipping Agent, Command, Skill, and MCP definitions.

I've created and tested against an example plugin https://github.com/ryanrhughes/opencode-plugin-example (keep in mind the devDependency is using a local ref for @opencode-ai/plugin.

Fixes: #6347 & #6013