Add backbrief-kit to plugins - #40

Adds a reference entry for backbrief-kit.

What it is: an orchestrator plans a goal into units with acceptance criteria and routes each to a specialist agent; a separate verifier reviews each finished artifact in fresh context against those criteria, receiving the artifact and criteria only and never the producing agent's reasoning; /handoff and /pickup carry project state across sessions.

Manifest: .claude-plugin/plugin.json, version 2.10.0.

Two notes so the entry is accurate rather than flattering:

• Tags say commands, not agents/skills. The plugin itself provides two slash commands at its root (/backbrief-kit:setup, /backbrief-kit:verify-install). The agents, skills and rules ship in kit-payload/ and are installed into the user's project by /setup rather than loaded as plugin-native components, so tagging them would put the entry in filters where a user would find nothing plugin-native.
• The licence is custom and source-available, not OSI. Free to use and share unmodified, not for resale. Stated in the license field rather than shortened to something misleading.

Repo: https://github.com/charlesashe/backbrief-kit