Briefing

Manage session-handoff briefings under `briefings/`. Subcommands: `request` (file a new briefing — collision-safe NNN allocation, YAML frontmatter, fresh branch + PR), `pickup` (claim an open briefing for this session), `done` (mark a claimed briefing delivered), `release` (clear a claim without delivering). The briefing schema, index format, and per-subcommand procedures for pickup/done/release live in reference.md — loaded on demand. Use when a session needs to hand a contextual problem to another session, or when this session is about to pick one up. Don't invoke for: single-PR-sized handoffs (use an issue), tasks the same session can finish (write code instead), or kn…