RFC: plugin architecture for custom agents (v2.0.0)

[PAMulligan]

Goal

The v2.0.0 milestone scopes a "plugin architecture for custom agents." This is the highest-risk scope item — we need a design doc before code.

Acceptance criteria

• RFC document committed under docs/rfcs/0001-plugin-architecture.md
• Covers: plugin discovery, manifest format, lifecycle hooks, sandboxing, versioning, distribution
• Defines compatibility between plugin agents and the built-in 48 agents
• Reviewed and accepted before any implementation work starts

Milestone

v2.0.0