Execute a chunk

Build the chunk in flight as the relay workflow's executor. With no argument it finds the chunk named in the repo's status file (docs/status.md by default); with an ID it runs that chunk's brief (docs/chunks/<ID>.md). Opens the chunk branch and a draft pull request, reads the brief's kickoff notes and the review-checklist entries they name, builds each unit with tests, runs the repo's gate, adds changelog entries where the repo keeps them, commits and pushes per unit, logs any decision the docs do not cover, writes the report into the chunk file, marks the pull request ready, and hands off to review. Use in a repo whose AGENTS.md has a "Relay workflow" section, for "/rela…