This document develops option 4 only: a full, extensible plugin platform built from declarative integration packages, supervised external plugin peers, and a capability broker.
The target is not a larger harness.json, an event-to-shell hook system, or an in-process Go plugin ABI. A plugin is a versioned participant that can declare contributions, negotiate access, receive structured context, perform asynchronous work, and contribute useful functionality without violating hrdx's PTY-centric model.
The design deliberately combines the richest parts of the earlier directions: