Write a new Architecture Decision Record following CONSTITUTION §7, wire it into the `docs/decisions/README.md` index, and update the status lines of any ADR it supersedes. Use when a change meets an ADR trigger in AGENTS.md ("When to write an ADR"), when a human says "write an ADR for X", or when called by `implement-spec` / `draft-spec` at the point a decision is made. Edits files — it creates the ADR and the index row in the same change as the code it documents.