Plugin API for custom AI commands + extensibility

## Overview

Allow users and the community to create custom AI commands for the Cmd+K panel.

## Prerequisites

- Cmd+K v1 + v2 complete
- Plugin system (`packages/plugin-api/`) stable

## Deliverables

- [ ] Plugin API for registering custom AI commands
- [ ] Import/export AI command definitions (JSON/YAML)
- [ ] Community command marketplace concept design
- [ ] Documentation for command authors