Agent Plugin System overview

Agent Plugin System as a cross-cutting topic - the plugin architecture for extending agent capabilities through installable packages. Covers plugin components: skills (slash-command units), agents (sub-agent definitions), commands (custom CLI commands), hooks (lifecycle interception), and MCP servers (external tool integration). Plugins are distributed as .claude-plugin packages via npm registries and installed via install.md manifests. The plugin.json manifest declares components, harness targets, template variables, and extra file sets. Supports per-harness install surfaces for cross-agent compatibility.