write-milestone-brief

Typical invocation points:

• User says "capture this" or "write it up" after a planning discussion
• End of a discuss phase before moving to plan
• User wants to hand the work off to another agent or a human teammate

<core_principle> SYNTHESIZE, DO NOT RE-INTERVIEW. Use what is already in this conversation. If a decision is genuinely missing, note it in Open Questions — do not relitigate.

REAL OUTCOMES, NOT TASKS. M###-CONTEXT.md describes what the user can do when the milestone ships, what scenarios must pass for "done," and what architectural decisions were made. It does NOT list tasks or implementation steps — those belong in M###-ROADMAP.md and S##-PLAN.md.