Add backbrief to Developer Productivity

[EvgenSmith]

Adds backbrief — a Claude Code plugin that turns call transcripts into a markdown vault the team owns: per-call digests with (MM:SS) anchors, decisions and next steps, plus tasks extracted and deduplicated against the live tracker backlog before anything is created.

• Real use case: generalized from 3.5 months and 200+ calls running in production before release.
• No duplication: nothing in the list covers meeting capture or team memory today.
• Tested: claude plugin validate passes; the kit ships its own test suite (render/smoke/e2e) and a publish gate.
• Zero npm dependencies, MIT (the optional n8n pipeline under pipeline/ is BSL 1.1).