Plugin SDK RFC for Claudius v2

[PAMulligan]

Goal

Design the plugin/extension surface that lets third parties hook into Claudius (custom tools, message transforms, transport adapters) without forking the widget.

Acceptance criteria

• Write RFC document covering plugin lifecycle, hook points, and security model
• Specify TypeScript interfaces for definePlugin()
• Document v1 → v2 migration path for existing forks
• Land RFC in docs/rfcs/0001-plugin-sdk.md
• Open follow-up issues for each implementation phase

Milestone

v1.2.0